The engine is the main question, since not really affordable to rebuild. A simple test is to remove the oil fill cap and judge the "blow-by" (keep a rag to clean oil spits). A little puffing is OK, but if a lot of flow, the rings are gone and likely the cylinder liners worn. If no puffing, put the cap back and remove the PCV tube. If no puffing thru that small hole, the engine is like new. If a good engine, $2500 sounds like a good price since the engine should be good until at least 300K miles. CA cars are almost never rusted.

I wouldn't worry too much about the seats. Most driver seats need rebuilding since the horsehair cuts into the springs, and springs break. Leather covers are expensive or un-obtanium (I need). I would consider using seat covers or eventually installing new seats from a newer junkyard car, as others have. The factory seats were too springy even when new, in my opinion.

Rollguy is right about the R2 compressor. It seized in both of my 300D's. You can read about it on GM websites. I bought Rollguy's Sanden brackets but haven't installed yet.