Definitely buy a beater... 'cause you'll trash the hell out of your first car, without even realizing it. I've had several vehicles, and I drive harder on each of my newer ones, but you can drive hard without tearing up a car, and it takes breaking a few things before you realize that...

For perspective... my first car was a 1986 Toyota Corolla... paid $500 cash for it, and I still have it (awaiting a tranny rebuild at home) and my sister is using that until she can BUY HER OWN CAR to drive. Just realize that whatever you get, you'll probably break or wreck, as most 16-18 year olds do... I never wrecked mine, but damn... I got a lot of DIY lessons from breaking stuff that never would have broken if I drove the car properly...