Waterproof would only be good if your idea is for it is "dustproof" since in would destroy eveything beside the servo (I.E. pointless really)..
Good servo choice:
http://www3.towerhobbies.com/cgi-bin...&I=LXFRS6&P=SM $30/steering
http://www3.towerhobbies.com/cgi-bin...&I=LXDEL9&P=SM $13/throttle and breaking
I perferably use karbonite gear since they are ALOT stonger than plastic gears and yet lighter than metal.
Whats your budget?