How much do you want to spend?
The best way to get a good buggy in this game is to get a pro kit, select a good engine and buy a decent radio all seperately.
the pro kit of any of those buggies will do.
Just remember though, its mostly about driving.
Mind you a good pro kit should be running long after your mates have retired with broken parts .

You really only have to watch out for the guy with the Losi, as the rex-x is a cheaper buggy with a not so hot engine and the 2-speed won't help at all off road.