If you buy this car and do the IMS and RMS, at 55,000 miles you may also end up replacing the clutch, pressure plate and throwout bearing, maybe the flywheel too or at least have it resurfaced. If he has owned it for 10 years does he have the service records - have the water pump, AOS, serpentine belt, spark plugs and coil packs ever been changed? And how long ago for each. Also how often has the brake fluid been changed for both the brakes and the clutch?
If you're negotiating in a friendly and reasonable manner there is no reason to not make offers and see what will stick. If you're on a budget, the best maintained and cared for car out there might cost a little more up front and be way cheaper in the long run.
There are lots of 986 Boxsters and Boxster S cars for sale, but as they say you have to kiss a lot of frogs to find a prince. In addition to local classifieds, Craigslist and Autotrader, etc. take a look at the classifieds here, at
Porsche Club of America and
Excellence Magazine. The independent garage you would have do the PPI on this car may have a customer or two that have Boxsters for sale.
Good luck and let us know what you end up buying!