If it is in good shape, $300 is a pretty decent deal. We recently had a thread/debate on whether a 1983 Trek 400 was worth $300. I was in the "not worth it" camp. This bike is much different. Well up the food chain, near the top of the line.
As far as upgrades, the sky is the limit. Read the various contributions to the STI vintage bike conversions.
Bike originally came with Suntour Superbe, which is really good stuff. Original components would not have been Suntour Sport. And are you sure you have that name right? Probably either Suntour Spirt, or Suntour Sprint.
Personally, my priorities would be:
#1. Fit
#2. Frame condition.
Component condition is not as important, they can always be replaced relatively cheaply. But if the frame is damaged, or the bike has significant rust, or the paint is in poor shape, fixing any of that is pretty involved.
Last edited by wrk101; 11-27-10 at 05:42 PM.