Being honest is the best policy.
We have a saying in the Classic motorhome forum; "The purchase price is just the down payment".
There are a lot of variables when purchasing one of these old queens, but #1 is;
Is it used regularly? If it is and maintenance is being performed regularly, then up front cost will be minimal. If the coach has sat for an extended period of time or maintenance has been neglected, then prepare to ante up big time.
Possible things to have repaired immediately before driving it anywhere.
Tires- DOT date them, over 10 years, or checked from sitting; replace, 9 tires including spare.
Fuel system: Ethanol is crazy bad for old rubber fuel lines and engine fires result; change all lines including over the gas tank. New fuel filters.
Coolant system: Radiator flush and possible re-core, new water pump, hoses, thermostat and coolant.
Braking system: Replace master cylinder, replace 30+ year old rubber brake lines as it makes a huge difference in braking. Replace fluid, lubricate and inspect callipers for seizing and pads.
Engine: New drive belts, engine oil and filter, sparkplug wires, sparkplugs, coli packs, timing; inspect for leaks.
Replace and inspect transmission fluid, filter and rear differential fluid.
Suspension: Front springs for correct ride height, steering crank bells, tie rod ends, ball joints, especially lower ones as they can let go. Inspect and grease all suspension points, remembering to tip off nipples before applying grease; don't forget driveshaft and hangar bearing. Inspect air system, compressor, air lines for leaks, front air bags; rear airbags for leaks and checking, (replacing these can be time consuming or expensive).
Lights, wipers, electrical.
If you cannot fix any, or most of this list by yourself, you can see how these queens can get expensive fast. On the upside; parts are fairly cheap unless buying Henderson SuperSteer stuff, however labour is not.
Per Mare, Per Terram and may all your campaigns be successful.
“It’s a recession when your neighbor loses his job; it’s a depression when you lose your own.” "Harry S Truman"