You need to search these threads and find the 100 or so other posts that ask teh same question.
Here is the skinny, get a car you can get parts for. Support your LHS and they will help you. Kyosho, Mugen,Thunder Tiger,X Ray, Jammin. All really great cars, all can win. If you go for the RTR cars you will have to upgrade alot of stuff quickly. If you save some money and buy one of the best then you will only have to fix what you break and that will be it. Just my .02 take it as you wish.