Need a lot more info- 500 bucks generally buys you problems the previous owner didn't want to deal with, and not much else. Of the 25 or so cars I've bought in my life, the most expensive was 750, so I know of which I speak.
Also (and I don't really know you, so don't take offense) I don't think a guy your age needs, or needs to think about, horsepower. If you're anything like I was as a teenager, you'd find plenty of trouble with 100 hp under your foot. Lost my license like six times before I was 20, driving worn-out Darts. Brakes and handling are where it's at.
I recommend a Volvo 240 or 740 station wagon. Great handling & brakes, bulletproof engines, built like tanks, and cops look at them and think "These are not the droids I want."