regarding the gears, don't worry as much about the spur gear the traxxas offering will probably be ok.
The key is finding a good spur gear to work with your combo. Mike offers numerous options and since the 5700 use 3.2mm/1/8" shafts you should have many options even at you local hobby shop.
Visit Brian G's calculator for help with gear selection (others here will chime in too I'm sure.)
http://www.scriptasylum.com/rc_speed.html