what 08 model to get

Well, i was wondering if i can get some info on some of the bikes you would perfer to ride-motocross wise. i have looked up on the internet for research on these bikes but i dont know which fits me. I've worked up for the past 2 and half years and finnaly made enough money to buy one and to be honest; i want to get into racing motocross. But, im not sure if its a waste of money. Because, i have heard that you can only ride a regular 08 crf 250r for example for 30hrs. and then you have to buy new pistons, etc. You see, i really dont have the money to be doing this all the time. I would really appreciated it if you can tell me what bike would fit a 170 pound, 6ft 1in guy. And if you could please tell me what i can do to prevent my bike from blowin or seizin and what would i have to be paying to fix it. thank you