He wants something legal to race with, and the Mamba definitely is not legal. Not only that, but the Mamba isn't smooth enough for racing either. I have owned two Mamba systems in the past and glad I got rid of them.
I have also owned both the XXX4G+ and a BJ4WE and the BJ4 is quite a bit easier to drive. It is also built tougher and easier to work on.
I suggest to start off with a spektrum radio system. This is so you can avoid the hassles of changing crystals and avoid interference. Get a spektrum pro system. Then get yourself a spread spectrum Novak GTB speed controller. It runs more efficient when used with the Spektrum or other 2.4GHz radio systems. A 5.5r Novak motor would be ideal in a 4WD buggy. I use the Hitec 5945mg servo that is very powerful and turns the buggy tires easily.
Get yourself a pack of loose matched 4200mah cells and you are in business.