I suspect I'll have to update this with some measurements once I get home and can take the bike apart further, but here goes.
I've got a '77 ('76 accoring to serial number but supposedly they didn't quite make these in the 76 model year) Schwinn Sportabout (steel frame) it's currently using SunTour front and real derailleurs w/a 10 speed setup.
Ignoring the technical difficulty, would it be humanly possible for me to convert this to Sora or better road components? I'll try to get some data on the bike to figure this out better, but this is what i'm guessing i've got to do.
1) replace hub (i.e. rebuild at least the rear wheel) (I have 27" wheels, is this possible?) (would it be cheaper to try to find pre-built wheels)
2) cold set the rear fork for a larger hub (tough, but after reading sheldon brown, I'm convinced I could do it)
3) replace possibly the entire crankset (I think i'm going to hit the most incompatability here)
4) replace shifters/reroute cables/replace brakes (pain in the rear but should be totally possible right?)
also, any idea (ballpark figures are ok) how much a more or less full set of sora/tiagra/ultegra components are going to run?
Your biggest potential difficulty is still easy...
If the bike uses a one piece crank, you will need to decide on which adapter to use to accomodate the new bb/crank.
Also, you can lace any standard rim on any standard hub as long as the spoke count matches and you get the right length of spoke... so you can stay with 27" if you want. You can also convert to 700c as long as your brakes have an additional 4mm or so of reach.
You will also have to do some drilling if you move to brakes with recessed mounts, and may need a claw adapter if your frame doesn't have a derailleur hanger.
You should be able to find all of the clamps and adapters needed to make the change a possibility.
If you really like the frame, you will end up with a nice bike... but if you are looking to save money in getting the best bike possible, it may be more cost effective to just buy a new or lightly used Sora equipped bike online...