what kind of track has this class? 17.5t (assuming blinky) seems really under powered for 4wd buggy. and dex410 is one of the heavy one in the class (not helping).
perhaps the track is very small and tight? or I'm sensing this class going to be a motor burning expert class.

oh, for gearing, best idea is to go to the track and see what others use. and calculate yours.