Right, in no set order

1. lead weights in the wheels.. remove tyres, buy wheel weights from you local tyre fitter. apply more to the fronts than the rears. I put as many as i could physically fit in the fronts, one full round less on the rears..

2. cut wheel foams.. while the wheel is off for the lead weights, cut the foams to create a 8 - 10 fingered star shape.

3. Lower the COG (center of gravity) in any means possible.. axial i put a small receiver battery on the front axle.

4. Waterproof electronics.. many web articles on this subject.. Google is your friend.

ESC selection.. there are 2 main options.. brushless or brushed..
brushed is cheaper by heaps and heaps, RC4WD Outcry is what i use, i picked up for about $60AUS delivered from america. Size is a slight factor (size of Novak GTB + BEC) but only raving review from me.

A good strong 35 turn motor is pretty good.. more speed, go for a 27T. You can go much higher in turns 55 - 65 T but pace is extremely slow.