Chris, an idea for the motor and controls...
Some washing machines are driven by huge stepper motors, one or two horsepower at full power. Of course they produce torque at low revs (even stall).
Now, the germ of the idea!
This is a sweet you lady young have in that machine and you want controls to be easy, safe and easy to learn.
SO...... make a set of regular pedals (rotary kind) and fit a multi position switch to the pedals and wire that switch to the stepper motor coils. As she turns the pedals the appropriate coils in the motor will energise, faster, slower, backwards, forwards, all as per natural pedalling but with the horsepower of the motor to drive the Toyrover. Stopping pedalling puts the brake on too.
The motor coils in the NZ made (now also made in other countries) Fisher and Paykel washing machine are full power at about 40 volts (and can spin 10kgs of wet socks to 1000rpm in seconds) but I expect 12 volts would be about right for that machine. Other marques make motors like that and some use the F&P motors.
This is what the motor looks like:-

The bit with the shaft is just the bearing holder, the stator is the bit with the coils and the rotor is the gray plastic bit which carried permanent magnets and fits around the outside of the stator.