speedknot

I just landed a great deal on a new SMT10 with all the goodies(not installed). I think the previous owner lost interest. In this package was a Castle Sidewinder 3 brushless set up that will be going in soon. Its got JC Renegades and I just installed Axial metal gears. My question is, what size spur and pinion would I be looking to install? I don't want it geared to be ridiculously fast. Somewhere in the middle I guess. It also came with a 3S Lipo, which I'm apprehensive about using. I'm thinking that a 2S Lipo will suffice, without generating over the top power. My 7yr old will be using this as well.