The big things I would be looking at for the age of the vehicles your looking at is that General maintance has been performed. They are new enough that "Age hasn't delt it's wicked hand" yet. Miles will probably be low.
vehicles of that age do not have servicable items like U-joints, ball joints, tierod ends etc. Sometimes the 2500's do. The 2500 was built as more of a workhorse.
When I look at a vehicle I like showing up early first thing in the morning. The goal it to get there before they have had a chance to hide things like weak batteries, hard start when cold problems, bad valve guide seals etc. It also means the engine cold and I can look and touch items that would be otherwise to hot to be touched if it's been run. From the top look at the belts. New belts good. Look at the water pump. If it's rusted...GOOD. It means it's been replaced. If it was the original pump then it would be painted. Some places will paint new pumps so see if it's a different shade or color or cleaner then the rest of the engine. It also means the thing probably has had at least some of the coolant replaced.
Pop the radiator cap. The years your looking at should have Dexicool (sp). It's red coolant. It should be fairly bright. If it's dark it's old or there is a possible head gasket problem. Green coolant should be changed every two years or so. They say the Dex stuff (red) is good for 5 years 100k......jury still out. The problem is coolant has chemicals in it to protect the cooling system from rust and corrosion. The green stuff is good for about two years. If it hasn't been changed then the system may start having troubles prematurly.
If you pop the cap and it's green then that is a concern. The red and green coolant should never be mixed. As long as all the red is out and it was flushed then not a problem the question is if this was done right or they just dumped in green and the block was still full of red.
Pop the brake fluid cover. Brake fluid is another neglected item. Brake fluid is recomended to be changed every two years. If when the brakes have been service, if it was a repitable shop, they will have been bleed. The fluid should be reasonably clear. The problem is again corrosion as well as higher potential for brake fade.
Brake fluid will absorb moisture from the air. When it does the fluid becomes acidic and eats the lines (both steel and rubber) from the inside out. It also make the fluids boiling point lower. That means it could have a brake fade or a complete loss of brakes from a couple hard stops or a long grade. The fluid will boil and the water will become steam. That can get you killed. If it happens, you will loose brakes. Reason is steam in the brake lines will act like a spring and you will loose the fluid pressure that actually makes the brakes work. That peddle may move 4 inches but the cylinder it actuates in the bake system will have only mover 3/8-1/2 an inch. It's not the volume it's the pressure. The 1/2 inch the pision in the master cylinder moved equates to about 500psi of pressure. Air (steam) will compress quite a lot at that pressure. Fluid will not.
Ok now I always bring a ratty blanket and wear old clothes. I also bring a couple pairs of latex gloves and a small flash light. I like to look at the bottom. Look where it's parked for any drops of oil. Things to look for is lots of oil on the bottom of the engine. A light coating...ehh might not be a big problem, Just a little seapage. The big place to look is where the transmission and engine join. Lots of oil there could indicate a failing rear crank seal OR a failing front pump seal on the transmission.
Look at the color, Pink is tranny leaking. A pan leak at the front of the transmission can also cause fluid to get to the same place. try to identify any other potention places it could come from. Those seals are very expensive to have replaced. Now just because oil is there does not mean it deffinatly needs a crank seal. Often a valve cover leak wil let oil run down the back of the engine and cause oil to be in the same location. Even sloppy oil changes can cause oil to be in that location. You can usualy look up their and see and often you can reach up there and touch the back of the heads and see or fell for oil. Valve cover gaskets are $25. Labor to install them could be $100 or more depending on how much stuff has to be moved to get at the valve cover.
Of couse also look at the condition of the the bottom of the truck. Rust, Signs of bottoming out from jumping curbs, signs of wrecks. If it has been hit in the rear often the floor will get wrinkled and they will hammer it flat. It's real obvious most of the time. Look at the frame near the bumbers for missing or new paint. That's indications of a wreck and the frame has been straightend. Uneven tire wear on all tires, side wall damage on the back side of the wheel. Caved in gas tanks. Lots of mud from being off road. Oil or fluids running out of the brake drums indicating failed seals.
Inspect the transmission for leaks as well. Axles, look for leaks, Shocks...yep leaks...if they have a film of oil on them they are short. Bilstiens will set you back $250 for a set of 4 not counting instalation and at the very least both shocks on the same axle should always be changed as a pair. Best to replace all four at once.
Look at the drive shaft. If it has greasable u-joints then look for grease slung on the bottom of the truck. This is usualy a good sign that they have been serviced. Grab the shaft near the joints and try to move them. if they clunk side to side they are shot. TIn good shape they should not have any perceptible movement. You may need to set the parking brake take it out of park and put it back in park to relive driveline bind. Rotate the shaft as well. If the parking brake is holding then you will have some rotational play. Debeing on the rear ratio it may be as much as a 1/8 of a turn with a 3.73 and a 4.10. gear. That's normal. What yo want to look for is tisting in the u-joint. Look at the cross and seals. Again if there is movement that's bad u-joints. That truck should also have a mid shaft bearing. It will be a two peice shaft and the bear will be in the middle. look at it and move it around. The bearing is in the middle of the rubber bushing. Look for cracked rubber. If the rubber is cracked it could cause vibrations and noise.
Before start up I like to look at things like the fan clutch. try spinning the fan. it should have some resistance. it it spins real easy it is either bad or has been started in the last couple hours. This clutch being bad will give you oveheat problems in traffic especially with the A/C running.
Pull the dipsticks and look at the fluid. Most people that sell a car will have the oil changed. Dark oil is not a real concern. It's a normal occurance due to blow by. now that said if it has a oil change sticker and it was changed 300 miles ago and it pitch black...walk. Transmission fluid is the one that they usualy don't change because it's $60 to do it at most lube places. If they do it tiemselves they can only change about 20% of the fluid by a pan drop. REAL hard to hide that they haven't been taking care of it. It should be pink. a little brown...it's got miles on it. not pink at all it's neglect or that vehcile has been pulling heavy loads often.
The 4l80 seems to be a pretty good tranny. I have not heard of many premature failures and the years your looking at it's not a new tranny. They first came out in 91 and have had some progressive improvements.
Many will tell you if you change the fluid on a neglected tranny it may be the preverbial "straw that breaks the camels back". It's true. The filters used in transmissions often do not filter as well as a oil filter on the engine. It doesn't need to because they are not dealing with blow by and the aluminum housings are a little less prone to forming condinsation. The fluid will carry some clutch material and even a little of the metal from the worn steels and it will help provide friction to worn clutches.
That said, I'm less affraid of seeing a little browning of fluid and hearing it has been 30k since the last fluid change then new fluid and no documentation that the transmission has been serviced as the manufacture recomends. Now again the years your looking at suggest reasonably low millage so your probably ok. Ask for the recipts. If no documentaion that it has been changed then ask. If it has 60k on it and it's still fairly pink I doubt you will have a problem.
On start up (with A/C of)f...crank it and jump out and look at the exhaust. If it belches blue smoke it has bad valve guide seal. The parts are not expensive but it's a labor intensive repair so not cheap. A little black smoke that disappears right away is a rich condition from being cold. not really a problem. Fuel injected vehicles don't do this often.
Also listen when it first starts and watch the oil pressure gage or light. You may hear a clatter that goes away as the oil pressure comes up. That noise is cheap oil filter with a crappy drain back valve. If you look under the engine and see a Fram or a noname filter from the local oil change place that's the problem. Use a good flilter like a A/C, Wicks, Bosch (rebranded A/C), AMsoil, Mobil1 and that won't happen or happen VERY little.
GM's high idle when cold to get the cat hot fast to lower emmisions. 454 seems to do this more then 350. That said let it fully warn and lsten and watch. A little shake of the engine cold is normal. Once warm it should be a smooth even idle. If it doesn't have a smooth idle it can be from several things. Tune up being most common. Bad compression being the worst. If the owner have reasonable documentaion of oil changes at the years your looking at I doubt it's a compression issue.
Smell the exahaust. Now don't get you face up in the tail pipe but smell the air. A sweet smell is coolant being burned off. BAD...that's a sign of a blown headgasket. Bet the coolant is really dark. Steam (unless a cool morning) is again a sign of blown head gasket.
Exterior.... does all the paint match? Only the best body shops get a nearly perfect paint match when doing a body repair on anything but white. The paint job is usually better then factory. Look around trim and widows seals for overspray or tape lines where the paint is on the seals. WHole front clip different shade then the doors back.....it's been in a wreck. Look at the fram, look at the core suport. A minor wreck is no big deal. It's just knowing how to tell if it was a minor wreck or a bad one. That's something that's hard to explain. THe biggest give away is front clip and front door(s) repainted. That would indicate a hard enough hit to push the fenders into the doors... That's a pretty hard hit......WALK AWAY.
The test drive. TURN OFF THE RADIO.......Who cares about the stereo. Your buying a truck, The tunes are secondary. If it's hot check out the A/C first. Does it cool well? do all fan speeds work? Slide it over to hot. SMELL. Sweet smell is leaking heater core and that can be an expensive repair. Odometer look like it's working right? any finger prints under the lens indicating a rolled odometer? Most newer vehicles have a tamper indicator. iff somebody has tampered with it silver or white lines will be visable between the digits. Digital cannot be reset easily. Pretty safe. In 87-93 you could actually pull a fuse marked Speedo......no miles would collect. GM now puts the speedo on on the ingnition circuit so no way to disconnect it easily and have the truck drivable.
9 0ut of 10 chance when you go from park to reverse or Drive or back from drive from reverse you will hear a clunk. If the u-joints are good......I wouldn't worry a great deal. It's a GM thing and most start doing this when they have over 50 k on them.
Does it accelerate smooth when you acclerate hard? ( I like to do this with A/C off windows up) any misses or ticking or pooping noises under full throttle? Indications of detonation from bad knock sensors or carbon deposits.
Were the shift firm? Steady cruise is it smoth? easy accleration were the shifts soft and gentle? These transmission are tied into the computer. Full throttle it should shift firm and hard. when doing a easy take off it should shift smooth and soft. It's designed that way. If it has a Tow button hit it and try a hard and soft start again. THe tow mode increases line pressure and firms up shifts to limit slipping. It will also hold the gears a little longer. It should be a noticable change. If it has a OD off button it should also do this but not quite to the extent of a Tow button.
Find a bumpy road. Listen for clunks and bangs. Pay attention to any shudders you can feel when you hear them. These are indications of worn suspension. When ball joints, tie rods, bushings and even shocks can and will make noised when they are worn. If it's quite or it takes a reall hard bump to get any noise that's probably in good shape. If it's really banging on a modest bump it probably needs further invesigation.
It can be check further but you got to know what to look for. If you hear banging it might be time for a pro to look at it and tell you what it is. The front suspension is quite elaborate and unless you regualry do automotive repair yourself you wouldn't have experaince on how to identify bad parts.
Now if you still don't feel comfortable ask if you can take it someplace to have it inspected. Places like Good year, Firestone and NAPA service centers will often do this for a reasonable fee. It's better to find out if you have a $600 repair needed before you buy it.
Now I;m not a goog negotiator so I'm not much help here. I would not nitpick the problems you find. I would just politly go "well a few things will need atention soon" and let them question what you find is what I have been told to do......I never seem to do it right. Some how the sell get on the defensive but it might be partly my fault LOL
Ok fingers tired.