What ever you can afford that you can get parts for.
some guys use the latest and greatest. Some use eight year old or older. You can buy a brand new X-ray but it doesnt mean you wont get passed by a TC3. Just get a good lipo, a Novak 21.5 and a decent ESC.
www.battlefieldraceway.com Racing videos at http://www.youtube.com/snoopyrc