Make sure you get a PPI done (Pre Purchase Inspection). Even though he has all the books and everything is up to date it is still worth doing (especially since he only drove it infrequently). Issues can arise when a vehicle is not driven enough or when driven is not given time to really warm up (and lubricate correctly, dry out all condensation in exhaust etc).
I love my '99. Even though I get bit green when I see the new models come out I also recognize that there is not much difference between those and the car I am driving (a few HP and some nifty bits but the real pcar experience is still the same). I also feel that I am still better off upgrading my car (with suspension, wheels, tires, turbo, SC etc) than I am to go and buy another new model boxster.
If you can get your hands on a low mileage 99 for a good price then I would do it (although Pedro raises a good point
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