Buying a cheap frame - one that was cheap when new - and then putting expensive modern componetry on it is like putting perfume on a pig. Upgrading can not always be done independently, either. Lets say you want to upgrade to a 9 speed drivetrain - you would need to replace the rear hub, buy a 9 speed cassette, buy 9 speed brifters, 9 speed crankset, and 9 speed chain. None of these parts would be compatible with the older 10, 12, 14 speed equipment you have. You might be able to get away with the old derailleurs depending on the age. So if you're going to spend that much money on upgrades, start with a good frame. Good quality steel, Reynolds 531, Columbus SL, etc. A 126mm dropout spacing will easily accomodate newer 130mm hubs. A 27.2 seatpost gives you many more modern options, but is really not a limiting factor. If you want fenders, it's easier if the dropouts have eyelets. You might find, as many do, that the older frames make a much easier and cheaper modern transition to a single-speed or fixie. Or you may find that you love the old stuff and keep replacing it vintage.
But if you want to upgrade, watch out for:
1. proprietary bottom bracket threading (some raleighs, for one) - look for 1.37" x 24 tpi or 36mm*24tpi. French and Swiss 35*1mm are also hard or expensive to upgrade, and will also keep you away from french threaded headsets.
2. Proprietary headset/ fork threading - Raleigh again here on some models, and I think Schwinn had some odd sized, too.
3. 27" wheels can limit your choices if you upgrade your brakes and change to 700c wheels.
You'll avoid most proprietary issues if you buy a good quality frame.
Sheldon Brown's website has a wealth of information. Look at it first. And have fun.