i would get a 10.5 motor, that has plenty of speed for both the vehicle you are interested in!! as far as servo the 1251 (I think that is the one) from savox is good but often needs a bec or cap connected to the receiver as it take lots and lots of juice to run it!!
As much as I really want a faster motor, I don't think my track would let me race it in the 13.5 classes.. A 10.5 would be a lot of fun though. And will look up the 1251.
I was looking through the servos on Amain and I noticed there's either high speed or high torque. What is better for off-road?