try taking the servo horn off the servo...turn on the radio and battery pack and then center the steering trim on the radio. Now center the wheels as best you can and put the servo horn back on.
sometimes if you have to have the steering trim set far to one side in order for the steering to be straight...the servo will have more travel to one side than the other as it is really more to that side when you are steering straight.

you can run a 5cell nimh or a 2cell Life pack. A lipo may provide to much voltage to the esc or servos.