Even with some nice options, which don't really add much value, that car seems way, way overpriced. For $15k you should be able to get a nice 2001 or 2002 S.
Kelly Blue Book isn't a source I'd stake my life on but it at least gives you a good ballpark. Trade In: ~$8k. Private Party: ~$10k. Retail (exc cond): $12k. Edmunds.com is another source and values there are quite similar.
IMHO, looking at eBay closed auctions is most telling about where the market is. There are really clean Boxsters listed and ratty, abused ones, and everything in between. Not many cars sell, but where the bidding stopped for unsold cars is pretty revealing. You'll see a clear value picture after looking closely at 20-30 cars. There are LOTS of recent threads on this board from 1st time buyers like you asking for opinions on cars they're thinking about buying. Read them all and learn before you jump in.
I get the impression you haven't seen/driven many Boxsters before, this being the first really clean one, and now you're smitten. If you want an S, go for an S. There's absolutely no reason to settle. There are so many of these cars around. Prices are in the tank. Never a better time to be a buyer.
This site is great place to start your research:
http://sites.google.com/site/mikefocke2/buyingorsellinga1997-2004boxster
Step back, take a deep breath, research, research, research, pick the brains of people on this site (opinions are like rear ends, everyone's got one), look at lots of cars, find the right one, get a Pre-Purchase Inspection (PPI), and if it comes up clean then "do the deed."
Good luck.