As has been said above, find your self a used roller and build it up your self....in any rtr you will need better servos and if your racing theres nothing worse than your pull start giving up on you
So you will need a sealed back plate and starter box on top of the rtr buggy plus a good servo.
you can find a good losi 8 2.0 roller for £120. so I reckon you will be able to sort most of it for around £350.