There are so many things to check, the main one being rust! The mechanicals are really quite simple.
If you go to view one then you need to be prepared to crawl around underneath and have a good poke at the chassis and front and rear crossmembers.
Engine wise, make sure you see it from cold, start up and make sure there are no nasty noises or fuel/oil leaks, look out for a smooth tickover and make sure there isn't clouds of smoke under acceleration.
Bodywork is almost always an issue, but panels are readily available so if you are any good with a mig welder or you know someone that is you should have too much of an issue.
