One of the best LOW COST servos you can get for steering are listed below, but if you are serious about your car then get the best one you can afford. I have listed some below, there is a guy here on RCU that sells servos that he backs up with a money back gaurantee, I am planning on buying some since they are so cheap, lots of people have good things to say about them. I'll link that below too. Def getting a battery pack is the way to go that way you can use the full 6 volts for your onboard electronics servos etc... I would recommend the one im going to list below, its a great price.
Venom Receiver 6V 1200mAh NiMH FlatRadio or Venom 5-Cell 6V 1200mAh Receiver NiMH Hump
http://www3.towerhobbies.com/cgi-bin/wti0093p
Futaba S3305 High-Torque Standard Servo w/Metal Gears 32.99 here is the LINK:
http://www3.towerhobbies.com/cgi-bin...?&I=LXDWF8&P=C
Hitec HS-645MG High-Torque 2BB Metal Gear Servo $31.99 here is the LINK:
http://www3.towerhobbies.com/cgi-bin...?&I=LXUZ89&P=C
Hitec HS-7950TG Mega Torque Ti Gear Digital Dual BB $139.99 here is the LINK:
http://www3.towerhobbies.com/cgi-bin...&I=LXWCE6&P=ML
Power Plus Pro high speed and torque digital servo:
http://www.rcuniverse.com/magazine/a...rticle_id=1053
Name: Power Plus Pro high speed and torque digital servo
Price: Battery: $48.95
-Standard case size
- 1.6 x .08 x 1.5 in.
-Coreless motor
-Metal gear
-Double bearing
Speed: 0.17 sec. / 60 degrees @ 4.8v, 0.13 sec / 60 degrees @ 6v
Torque: 180oz/In. @ 4.8v, 208oz/In. @ 6v
Temperature range: -30 to +60 C
Dead band width: 4usec
Operation voltage: 3.5 - 8.4v
Connector wire length: 300mm