Like Don said, have a local dealer do a PPI on it, top to bottom. I would not buy any car without that. If the seller says he's offended, he's hiding something.
If you are going to drive it regularly putting more than average mileage, then don't cut it close budget wise unless you are willing to do some of the work yourself. Regular maintenance is not cheap, even at a Porsche specialist who's not a dealer.
At a shop here are the things I've had done and costs:
1) 30K service: $800+ at dealer (Oil, oil filter, air filter, cabin filter, spark plugs, check serpentine belt, check hoses, check/lube top)
2) 45K service: $250 at Porsche specialist shop, not a dealer (Oil, oil filter, minor checks)
3) Quoted $1400+ to replace coolant reservoir in rear trunk. Did it myself instead for $400 total parts, coolant, hoses.
4) Replacing all of my rollers in the serpentine belt line. The bearings are all wearing out and making racket. Parts are $150 w/ me doing it myself. Since I got the parts at Sunset Porsche, your local dealer will probably charge what mine does and that's $280 for parts. Even though it takes about 30 minutes, it's probably a 2 hour charge in labor so add $240 for a dealer to do it.
5) Replaced ignition switch. I was quoted $400+ for this job at the dealer because they will only put in the revised switch which requires the new key tumbler and re-keying it to match your keys. I could replaced the switch with $30 in parts in 30 minutes so I did it myself.
6) Quoted $100 for just an oil change at a local shop.
7) Tires last 30K miles for me. It will mostly depend on your driving habits. I use Kumho tires at a cost of about $140 each installed.
8) I saw on another forum where someone was quoted $4K for a clutch job at a dealer.
They have a long period between required service, but if you don't check things like the top joints, etc. between times you will end up with problems over time.
If you go to a dealer for service, it's $100-$120/hour labor. At a Porsche Specialist shop it will be $70-90/hour and you should get references from other owners before taking your Boxster in to them.
Most other mechanics will not touch a Porsche. Even if you can find one, you probably don't want them tinkering with the car. When I told my friend who is also my mechanic for the other cars I wanted to get a Boxster, he said he would not touch it even for a friend.
Good luck in your hunt!