3 Racing out-drives are great, but if you're short on cash 1 set for the back should be OK and as for ceramic diff balls...
Click Here. Even with postage, they are dirt cheap.
The alu suspension mounts are a must (unless you're not ever going to have a crash
) and the slipper, it should be pretty tough after those.
The trf shocks are nice, but not absolutely essential.
The next thing I'm gonna buy to make my Durga bullet-proof is some Hard Hex Head Ball Head Connectors, as they seem to be the weak link now :/