Good choice in car, quality is really top notch. You may need to get the 2.3mm roll bars and some softer rear springs though as the kit ones are the hardest.
The 1258 should be good enough for steering, but did you used to use in a nitro buggy? I had a 1256 in mine but it caused all sorts of problems with the receiver browning out even just by quickly moving the steering off the ground.
If you go to the Servo Shop, link here
http://www.servoshop.co.uk/ you can get a brushless Futaba servo for a similar price of the Savox. Even my Orca 1/10th speedo can power that and it has similar torque to the 1258. I tried my 1256 on the Orca and there was no way it could power it. The 1256, 1257, and 1258 are the same series just geared differently.