One big plus with going for a Mugen 6T is you do not need to spend/waste money on making the truggy competitive with custom option bits, this gives you more cash for the other items you need to race.
A $600 budget will get a decent 2nd hand truggy and new motor, GO, Alpha etc but your going to need to spend at least that much again on radio, servos, battery and tyres, and as the guys have said buy the good stuff and you won't regret it.