Not too sure if it is cheaper then HK, I was looking at the scx10 RTR Honcho, sale for 329 plus free battery and $50 off coupon, so$280US. (Plus shipping)
If you mainly run on trail, 27 better. More climbing may choose 35. They are fairly cheap anyways. Also, pinion gear (teeth) is important too; this will affect the speed and torque.