Let me give you a little advice. I bought a boxster four years ago wanting to do exactly what you want to do... mod the car right away. Boy, am I glad I didn't do that!
1) Save up and buy an S. No one says "I wish I had bought the smaller engine." I really wanted a 2000 S, but settled for a 97 with a 2.5 in it. The 2.7's don't have enough power either if you ask me. However, the 3.2 has a lot more torque and HP and is well-suited for the curb weight of the car.
2) Buy the color car you want and then drive it for a year before you mod it. You'll discover that there are repairs you have to make to any used boxster (coolant tank, oil filler tube, RMS leak, radiator fan crapping out on you, etc.) It would be foolish to put a couple of grand into your car wishing you'd used the cash to keep it running.
3) The body kit, by virtue of the low price, is probably not polyurethane. The last thing you want is a fiberglass front bumper cover. The very first time to scrape a curb or someone backs into you or you tap the side of the garage it cracks.
4) I hope you have a love affair with car wash soap and your sponge. Black rims are always in need of washing with all the brake dust.
I think buying a Porsche at 21 is a great idea if you can pay the horrendous insurance premiums for being under 25. If you haven't looked into that, you should so you have no surprises that first day of ownership. I sure wish I had the money and wherewithal to buy a Porsche at 21
