RE: RS4 engine
depends. If you are just bashing, with an occasional local race thrown in - i would recommend the OS .15 CV-RX
It is resonably priced - and is one of the most powerful engines out there. Top it all off with legendary OS reliability, and u cant lose.
Other good choices would be:
OS .12CV-RX
Fantom .15FR
Fantom .18FR
HPI .12SS