You can upgrade the radio, because if you couldnt there would only be 6 trucks per race
Futaba Fasst is a good 2.4g radio, easier to use than some others, and is not that expensive.
If they allow lipo batteries, buy one. Nimh will work fine, but it beats upgrading your batteries later down the road.
As for parts buy some shock caps, body clips, front a arms, and a front shock tower.
Just my .02