For the engine, that depends on how much you wish to spend?
Low budget: Hyper 8 port, SH .21 (sportwerks,LRP, Nosram etc), STS .21.
Medium budget: Axe rossi sport, Axe rossi mamba, Go .21 (5 or 7 port)
High budget: Axe rossi cobra, O.S. V-spec, Ninja .21, and many more.
As for upgrades on the buggy, Obviously CNC knuckles and chassis braces, CVD's all round, 6V receiver pack, steering servo. Other than that just replace broken/worn stuff with the pro version.
Got to admit I have never had any problems with mine, though mine was a kit so didn't come with that engine and radio (and i built it myself with loctite ).