Of all the things mentioned here the #1 issue you MUST look for is: RUST
I'm not talking about surface rust or the little bits of flakiness here and there and especially not the rocker panels which we all know will probably be either completely rotted away or just a paint shell. I'm talking about the rust that compromises safety and can lead to thousands in repairs in order to pass a safety inspection. Older model Expys are NOTORIOUS for rust issues. Before even taking a test drive....
Check:
- Radiator mounts up front. These are notorious for rusting away and taking the front body mount with it.
- Body mounts - ALL of them - especially the center ones below the "B" pillar and the rear-most ones, up behind the the rear fender wells.
- Inner body rail and all crossbody support members. These can rust away completely leaving a crunchy, unsupportive mess that you can push a pencil through.
- FRAME! Check the entire frame and crossmembers with a hammer and a screwdriver. Lightly rap all along the entire frame, especially the rear. If you get a dull thud or a dent, use the screwdriver and see if it pokes through.
Not sure or not comfortable with this then get a pre-purchase inspection and make sure they check what I mentioned above.
Run away from any Expy with structural rust issues unless you are good with a torch, have a friend that is, or you have deep pockets. The radiator support mount repair is upwards of $2.5K because the entire front of the the truck has to come off, the center front body panel has to be cut out, ground down, and a new support panel welded in. Body mounts aren't too expensive as long as you don't have to replace too many. Inners and body supports, depends on parts and how bad the rust is. Frame.. fuggedaboutit! Most shops nowadays won't touch a frame repair. DIY if you can hide it from an inspector, otherwise a frame-off restoration, IF you can find a donor frame, would be at least $5K for a vehicle that likely books below that.
I picked up an Expy that I only took a quick glance up underneath, dirty, but looked OK. Boy was I wrong. Later, AFTER buying it... I discovered it had rust through on all kinds of places on the frame. The rear was so bad you could put your finger through it in several spots on both sides. and the rear cross member that ties to both rear sides was rusted through so bad that I was surprised the rear held on. I wouldn't tow anything heavier than a canoe trailer. I was afraid that if I got hit from the rear that the entire rear would just drop out. It would never pass a safety inspection and certainly wasn't able to be resold for anything but parts. Worst part... it had a current VALID safety inspection... so either the inspector didn't even look at the frame because rust that bad would have shown up years ago OR the previous owner knew a "guy" that passed it. Either way, don't trust the ol' "well it has a valid inspection" routine.
Good luck and good hunting