First identify what the class is you want to race. I would go with what is the largest class usually at your track unless you find a type of vehicle raced that you really like. Look to make sure that the class has at least 1 full heat of vehicles regularly. You don't want to be in a situation where its a 3 man show as usually someone breaks and its not as fun.

For nitro, the easiest probably would be 2wd stadium truck since there are less moving parts but can be more challengine to drive. There are good RTR stadium trucks out there, namely the Losi XXX-NT RTR.

If you want something bigger, CEN's Matrix RTR buggy is very nice and includes a very good radio too. There is a review on this site about it.