.21 will be the only race legal engine. You can choose anything from $100-$500 but if you are new to nitro I would suggest you start with a cheep engine until you learn how to tune and maintain it properly. Stick to $200 or less. Some great choices for you in that range would be:
Novarossi N21BF
Hyper .21 8 port
Force JL .21
Alpha orange head
go, 5 or 7 port