http://www.986faq.com/7-0/schedule.asp
For the most part, the 15k service is a glorified oil change. Mostly it's general inspection of belts, hoses, etc. Basically to try and catching anything that might need adjustment/replacing premeturely. It checks obvious things like headlight operation, horn operation, tire pressure, leak checks, etc. If you are mildly competent, you can do this yourself.
30k is the 15k service, plus a more thorough inspection and replacement of things such as engine air filter, oil filter, and spark plugs. This will check such items as brake pads, clutch, polyrib belt, exhaust system, etc. I've heard that this ranges from $600-1000.
If the car hasn't had the 15k service, but has had oil changes and new tires, then that is not bad. However, it's in your best interest in any case to get a pre-purchase inspection (PPI) done by a Porsche dealer. Also since it's so close to the 30k service, you might want to either ask to have it done before you take the car (and adjust the purchase price as you see fit), or just plan on dropping at least $600 extra to have it done once you buy it. Granted you can replace the oil and filter, air filter, and spark plugs yourself and then take it to the dealer for the remainder of the service (will save you some big bucks).
I think you've got a good car, but be careful. The PPI is your best defense against anything that would have been caught had he done the servicing and needs to be fixed.
How does the car run? Does the clutch feel loose or heavy? Is there any play in the shifter or the e-brake? Does the engine idle smoothly with no roughness or excessive vibration? Do the brakes feel solid and squeal-free when braking hard? When you do a test drive, does the car temp come to just at or slightly above the 180 mark after the engine is warm?
These are probably stuff you've already looked at, but never hurts to bring them up again.
