The CAT SX is a good car and the one on the sale section looks great. This is what my son and I have. But don't fooled into thinking the once you've bought a buggy that's the end of the expense. You son is going to break it and the accumulated cost of wishbones, shock towers etc adds up. Believe me! Plus tyres, broken wheels.
As for batteries, Lipo are great. the Intellect brand you can buy from HK are great and half the cost of batteries in the UK.
Just trying to be helpful...
PS there is a big difference between the old CAT SX and new CAT SX II. The newer version does not have some of the aluminum part of the suspension as far as aI can tell.