ORIGINAL: Foxy
Anything over 160oz-in of torque at 6v for steering. You will also want decent speed as well. The TowerPro MG996 is a cheap option, but you can get much better if you are willing to spend more. My favorite 1/8th steering servo is the XP DS1015, but it's a $90 servo. If you ask me, you shouldn't go cheap on the stuff that makes sure you stay in control of the car.
10-4 on the not going cheap for servos. I come from a history of flying 90 sized helis 3D. Last thing you want is to go cheap on something that ultimately controlls your investment.