First check your linkages in the car to make sure nothing is binding, it should turn freely by hand.
then check to make sure you are charging your receiver pack to its full capacity.
just about any cheap servo should turn your wheels at a stand still, basically, the more expensive servos will be faster or have higher torque, mainly for monster trucks or 1/8th scale.
however if you wanna spend money, go with a KO brand hybrid gear servo or high end futaba, or JR.
if not the futaba s3003 servos will work just fine and they are cheap.
hope this helps