In a trailer of that vintage I think you should always expect to do some work whether it is frame, skins, wiring, pluming, etc. If you are knowledgable in these respects then have at a trailer that looks good from the outside. Even if the frame is in good condition there are design flaws that you can rectify yourself and rest in the sense that you are doing the trailer good by reinforcing or better yet repainting to prevent further rust damage.
The fact that you're looking at a 70's long puts you in the category that you are in for some work. These were prone to a lot of problems that others weren't. If you get one for a deal...lets say below 2500.00 or less and there are MANY out there expect up to or even more than 1000.00 just to get her home safely in tires, brakes, and possibly new axles depending on how far you want to tow.
Inspectors are very helpful but if you are taking a trailer apart you are now the inspector and you'll learn very quickly that if you're a weekend wrencher that it could take no less than 1 or 2 years to come close to being able to use it.
Owning an AS is like rock climbing. you'll find out a lot about yourself and what you can accomplish. These forums are here for people to express their fears and for others who have gone through the pain and reward of doing it themselves. If you give up quickly in life the AS might be the perfect thing to get you over the hump of never say die.
just be aware of what you have to understand when you work on your trailer. Welding, riveting, glass repair, 12v
wiring, 110 v wiring, propane systems, water pressure, plumbing, carpeting, wood flooring, cabinetry, abs repair, axles, brakes hydraulic and drum, brake actuators and controllers, AC, vents, caulking, upholstery, weight distribution hitches, towing harness and wiring (especially with 70's trailers since they switch a lot of the wires) and the list goes on and on.
If you've done home projects or even built your house it will help but if you've rebuilt your trailer then plan on building your house...YOUR HOUSE WILL BE A PIECE OF CAKE!!!!!!