Old mtbs are great if you don't mind smaller wheels, ride them myself. But something else to look out for are early 700c hybrids.
I'm planning to start on this 1990 Giant Innova. I think I'm going modern groupset but that could change. It'll take a 700c 45c knobby, has not to light full cro-mo frame but strong and comfortable ovalised down tube. Should look like a current bike when finished.
Also planing on drop bars. This old hybrid has a shorter top tube to make it comfortable. Old mtb's don't.
