Welcome to the Forum!
Maintenance costs aren't too bad, unless you need a repair. My 30k mile service was about $600-800 (I can't remember), oil changes at a my mechanic are about $100 (it takes 9 quarts), and tire wear seems to be fair.
I drive fairly spirited, and my tear wear is probably better than what most people are quoting here. My snow tires have about 10k miles on them and the tread is still very good, I think I can get another 20k miles out of them. The rears do go faster though.
I would recommend you read as much as you can and look at as many Boxsters as possible before making a decision, good luck!