So I've somehow gotten it into my head that I want to find a crappy 20 or 30 year old steel road frame to use as my commuter/beater bike. Perhaps a 10-speed, or perhaps a single speed; I suppose ideally I'd like the option of going either way. I've seen plenty of old/refurbished Raleigh, Schwinn, Ross, etc bikes advertised on Craig's List, and I think at least to start out I'll get a complete bike rather than just the frame (since I have zero experience building up a bike from scratch).
But also ideally as things wear out and/or I get excited about this project I'd want to be able to replace anything with modern components: certainly the wheels, bars, brakes, saddle, & drivetrain. Plus I want the option of adding fenders. And maybe a rear rack.
So my question is:
Given the above goals, what sort of things do I need to look out for when shopping for this old pig of a bike? Rear dropout spacing? Bottom bracket size/shape/thread? Steerer diameter? Seat post diameter? I'm just tossing out guesses here, someone please point me in the right direction.
What are the most important parameters which, if I overlooked, would pigeon-hole me into not being able to upgrade one of these old bikes? Thanks.