[dsraven]

look for the best truck you can find. check several out and you will get a feel for where the rust commonly is hiding. remember these are old trucks and have had lots of time to have been in accidents and/or have body repairs done. lots of do it yourself stuff done over the years. don't take anybody's word for it being rust free and never in an accident, they may not know the history. lots of guys simply repaired the damage themselves. look in, under, around and over the trucks. check the roof drip rails, the floor, the cab mounts, the cab where the fenders mount up, rear cab corners, hinge pillars, door bottoms, box cross rails, rear fender mounts, tailgate, floor under fuel tanks and the actual fuel tank, frame, running boards etc for rust, damage, body filler, sheet metal screws/pop rivets, seams where there shouldn't be seams, that sort of thing. basically, check on here for the common rust repairs in build threads and you will seethe common issues and how things are put together. also check the aftermarket suppliers for common repair panels. that will give you an idea of where to look for rust.
compare you skill level to the amount of work required on whatever truck you get serious about. there also may be a member living close by a potential buy who may be willing to give an opinion before you purchase.
sit down and write up a list of what you want in an old truck. then post it on here and ask how you turn your potential buy (supply pics) into your "want" list. there will be guys who have already done the list before and will have a good idea of what you will need.