that's a pretty good price on a used Si. My last sol was a 94 S. When i got rid of it, it had 176,000 miles on it and was still strong even though i didn't take care of it for shit (i didn't really care at the time...

i was a bad monkey.... ). check your local yellowpages for a company like carcheckers that will come out and inspect the car from top to bottom, inside and out (the even do compression testing and timing checks). If it checks out with no major problems, it's definitely worth the $100 the company is going to charge. (if you can't locate a name, call your local dealership/used car lot and find out who they recommend. go under the guise that you plan on buying a car from them...)
Repair costs = CHEAP ! even a trashed tranny is only going to cost ~$1000 to replace with a low milage used. the A/C unit however...

about $1300.... most parts are pretty cheap, but there are a couple of exceptions.
for example : you might wind up spending part of that $3K on some new roof seals. Check them for leaks VERY carefully (especially for damp moldy smells under the seats). Seals run about $1200....
D series engines are good engines for one REALLY good reason: b/c everyone wants the DOHC B-series engines, nobody wants the D series. if the engine completely eats itself, it'll only cost about a $500-1000 for a 30K mile used engine! Think about it, <$5K for a car with a 30K engine..... not too shabby huh....
yeah, it's not a DOHC, but there's plenty of time for that later! Good luck on your search and keep us posted if you buy!