Dave,

Buy a sabretooth and 2 Gimson motors and boxes and save yourself a lot of grief.

Mini mighty mouse has steering wheels but also differential drive and took years to perfect. The only reason the front steering wheels are still on it is because it gives precise steering, and it is there. The servo used is Titanium with Titanium gears, output spline and connected to a belt drive sprocket that has a separate overhead support. Cost well over £100. If it goes wrong the wheels will sit at 90° and you are dead in the arena.

If you can race RC cars then you will take to differential steering in no time at all.

Trevor