Kyosho st-rr truggy build help

I am starting a build. turning my st-rr into a bl menace. It is actuly a r converted to a rr. I just bought a 2200mmm and I think I am going to use either a 6s or 5s battery. I have a metal spur in it now for the nitro day. NOt sure what diff fluid to use becuase electric is a whole new animals for me. Also what pinion and spur combo do i need. WHy do people switch to plastic spur gears? THanks for all the help!