Just FYI, to my knowledge there isn't any front sway bar that has been made for the Boxster that doesn't have hinges on it (basically defeating the purpose), because there isn't really a whole lot of space in front to put in a bar, since the towers are covered by the plastic panels in front and any bar would have to cross over the battery. So unless you manage to find/fabircate a bar that's going to be a solid piece that will fit in there without having to rip anything out of the front trunk area, I'd say don't bother, as anything you put in there isn't really likely to tighten up the car's front end. You're probably going to find yourself more concerned with tightening up the rear end of the car for autocrossing.

Now that aside, if you have your heart set on it, then it doesn't matter what anyone on here says that isn't "go get a Boxster", because your mind is already made up. I wouldn't come looking for reassurances from us, because it doesn't come off as the most financially sound car choice for daily driving and autocrossing for even some 30-year-olds, let alone a 17-year-old. And that's not just to use your age against you for a cheap shot, because your situation isn't really the same as your typical 30-year-old, but you're also not raking in more dough than Bill Gates, I'm sure. ;) People's initial reaction is to try and save you the trouble and experiences they've had with their choice to dive headlong into Boxster ownership whether they knew what they were getting into or not. We're not going to tell you the car's not worth getting, obviously, just that you really want to make sure that you can afford to own it comfortably.

Plan it out, do some number crunching. Look up what parts would cost you (and go for the higher end of the price ranges) to get an idea of how much, at worst, normal wear and tear parts would cost you at cost. And also look into what options you have for acquiring some parts that aren't normally the norm for parts that wear/fail, and how much those could cost you, be it at cost or otherwise (again, at the most).

Also keep in mind that you're probably going to have to sink a few grand into the car right from the start, unless you find a real gem, to have it ready for regular DD and AX duty, so take that figure into account, too.

So go through and realistically plan it out so you can know roughly how much money you'll need to have on hand for typical maintenance, with a few thousand on the side for any unscheduled maintenance, and a few thousand right up front for anything that needs immediate work when you purchase.