My suggestion: look at the lugs and drop outs. A nice frame will also have a tubing sticker, like Tange, Reynolds etc. If the drop outs are stamped (not forged), it is a cheap frame, probably made of gaspipe.

http://www.sheldonbrown.com/gloss_dr-z.html (see drop outs)

Long, cast drop outs... and a top tube that fits. Many older frames were a bit taller than modern bikes, so you might find you have a little less standover. As long as my boys clear the TT, I only care about the TT distance.

If the frame has been stripped, it is nice to find one with a headset (if it is any good) and seatpost bolt (like a real one- not a hardware store special). If you need a brake or too, it is handy to still have the sleeves intact. Those details aren't deal breakers, but they can be a headache if they are missing.

Finally, when looking for something "vintage" it helps to have a serviceable fork. Depending on your frame size, it can be difficult finding a threaded fork that fits (these aren't the one-size fits all threadless forks of today)- and buying a used fork can cost as much as the frame.

That's my two cents.

I prefer a 126 rear spacing- since both my fixed gears have that and can share wheels. These frames are also the newest, and can generally be found in better shape. I also like 700C wheels... it is easier finding deals on tires and tubes in this size. Its your call what you want to do with a crank and crankset. I prefer using a new cheapo Shimano sealed square taper mtn bike BB, scrounging for a 130 BCD crank, and using a 3/32 road drivetrain. I like to be able to interchange parts between my bikes- mixing and matching cogs and rings.