It's up to you man :) All I have broken is 2 front arms... (diff pinion was my fault) and for 4-6 months use, that's pretty sick as far as i'm concerned!
I can't remember anything else off the top of my head anyway! ;)
tbh, if I were to go back, and buy again with the knowledge I have now gained from owning the caster. Either get an ECO or get the EX1.5R Pro, this will last much longer than the RTR, it's got all of the hardened and lightened peices you will need and will be just as good as the mugen.
Either way, your son will be very happy with the performance of the kit you choose. personally, if money wasn't and important factor and I had a load of it to spend, i'd get the eco, because of my LHS stocks the parts in store, (caster's come in next day on order which is excellent too). and I light the motor mount on the eco, but i will soon be getting one of those similar for my caster :D

