I'm in the East Bay, near Walnut Creek. I'd be happy to let you drive my car and we could talk about 928 issues, but I'm not that well versed on early models. There are common wear and maintenance items and fluid leak points on all models, not unlike any other car (balljoints, tie rod ends, suspension bushings,motor mounts, pan gasket, cam cover gaskets, front and rear main seals, wheel bearings, trans mounts, steering rack, etc.) and pretty well known electrical weak points (relay, relay, relay) along with some front wheel alignment nuances that are often screwed up. If the car starts easily, runs smoothly, idles at the right RPM, has stable manifold vacuum, has good power, drives straight, handles well, is dry underneath, feels tight with few rattles, doesn't make any odd noises other than "bwaaaahhh", has good paint, a clean, intact interior, doesn't grind second gear if manual or slip between gears if auto, it's worth going further. In fact, that would be rare. Most 928s of that age have some issues. Then it becomes a matter of determining whether they are major. Any cosmetic issues are going to call for major dollars unless you can live with them as is, so good cosmetics (intact dash, pod seats, paint) is a real big plus. Compression and leakdown are always a good idea, but the engine is pretty durable (my car is proof), and if it runs very well and is low miles, you might forego that, but there is a shop on Campbell (CT Automotive) that has a good reputation for servicing 928s where I think you could get a PPI with that included.
Anyway, let me know if you want to come up this way sometime soon.