Like Rob mentions the RB6 is very strong. The steering rack is probably the most vulnerable in my opinion (although it still took me some effort to break it) and a lot of people have changed this to the Exotek version EX1258. Kyosho have their own alloy option coming out some time very soon too which should be very nice!
A rear bulkhead UM724 is also worth keeping in the spares box along with the usual pointy out parts.. But, the wishbones and hubs are pretty bullet proof. It takes something pretty bad to break those..
If you need anything just ask.. we have some starting setups on our Facebook page that might be useful.
Kev