How much driving experience do you have? I'm guessing not much since you're talking about buying a car... You could get a used one (but be very careful, in my experience used cars almost always end up costing more than new ones), but I'd recommend a new one, and especially if you're racing, then one you have to build yourself. I then suggest you bash around with it until you are really comfortable with it, then head to the racetrack and watch them for a couple of weekends, try to learn as much as you cna, then when you think you are ready, head over there on a practise day and try not to get in the way of the fast guys. Soon you'll be ready for your first race. If you have some experience please let us know, and apologies for my assumption...

EDIT: By the way, don't be afraid to post questions here, this is a good thread, ask away. There's no need to create a new one for every question.