I bought a 1976 El Camino in 1983. I was living in Lubbock TX at the time so I did not have any problems with rust. My major problem was with wheel bearings, exhaust system, brakes and shocks. It probably had those problems before I bought it though. Since it was only a car to get to work and back and haul stuff with I didn't worry too much about it.
My stepson blew the engine (he was hot rodding with some friends) and we had to get rid of it. I had a 350 with 4 barrel carb so it hauled a$$ pretty easy. As orboater says it sure could smoke that one tire! I also had glass packs on it so it sounded pretty mean too.
All in all I didn't have any serious problems. Up north in the snow and slush you may have troubles. Light on the rear end and if you have an automatic the rear wheels seem to spin with the brakes on in the ice. That can be real fun!
Tell your son to take it easy if you do buy it.