The frame on my commuter is an 80's Specialized. If you are buying one in mostly-stock condition, here are some things/components to know about.

Rear derailleur will be old, but many other old derailleurs are out there, and will be replaceable. You can still find new versions if you look hard. Chains and cassettes are readily available.

Bottom bracket is square-taper and non-cartridge. However, the threads will fit nearly any new bottom bracket you wish. Mine has been upgraded to a square-taper raceface BB that is still available new from raceface if you wish.

Steerer _may_ be one-inch. This is one of my biggest obstacles, because I can't upgrade the fork without a lot of looking. If you buy in the mid-to-late 90s, it will probably be 1 1/8, and will be easily upgradeable.

Identify other parts. My bike's rear brake is a U-brake style that is tough to replace and can't be upgraded/changed. It's also mounted to the chainstay, which happens to be the worst palce you could put a brake for a single-speed conversion. And my bike has been converted.

But, overall, it's an excellent vintage of bike, and should be easy to find parts for/maintain.