Need moar details, but sounds like a decent deal if it's not trashed.
If the fenders and header panel aren't torn to shreds, and it runs, it's probably worth the money. Non-Si models are carb'd and somewhat anemic, which is not to say you can't go .040 over, slap a set of integra pistons in and solex or weber sidedrafts on and embarrass FI guys. Carb'd models have a rat's nest of unnecessary, emissions related vacuum lines, which can often be the culprit if it's running rough.
As bravenrace said, rust under the rockers is also an issue, as well as the torsion bar mounts AND the panhard bar.
The only "ridiculously easy" swap is another EW/D15A3, though the D16A1 from a 1st gen. Integra isn't too bad. Anything else will require pretty serious work.
Some folks on redpepperracing.com will argue that it does nothing, but I've seen with my own damned two eyes a ~10hp increase on Si's by swapping in 1g integra injectors and ECU.
The Si's also love a E36 M3 ton of advance, and a bump in fuel pressure.
Motors as well as 5-speed transmissions are generally rock-berkeleying-solid, but they are interference motors, so inspect/ask about the timing belt.
Parts used to be plentiful, but are starting to get more scarce. Importers don't generally bother with anything pre-88 anymore, so grab free or cheap blocks, heads, complete motors, etc. when you can.
They're GREAT, tossable, fun to drive, easy to work on cars, and I hope to own and drive one till the day I die.