If your undecided, make sure you are completely committed before you get on the track. It's pretty much an all or nothing deal as we take our lives in our hands(and our other track mates at the time)each time we go out...trackday/practice or otherwise. That IS the reality of our sport! You say you're 18, so the mental preparation of that alone will take some time. I think most of use thought we were invincible at 18, but at least you've determined that street riding can be very dangerous....so I'd say your over half way there....thats a good thing!
I would suggest, as others have, for you to find a smaller displacement motorcycle and all required riding/safety gear in your budget, then spend some time with a CMRA mentor. I certainly would not recommend anyone, who has not spent a considerable(years) amount of time on a larger displacement motorcycle, to buy and try to start their racing career on one. In addition, budget for your first "get-off"....I'd say about $350-600 would cover things like used rearsets, bar, maybe a piece of bodywork, gloves, etc...that amount won't cover all of these at once, but maybe one or two of them, so you can get back on relatively quick.
Good luck!
Derek Thomas
Desert Rats #47