Ben, how many wheels do you want to use?
And what size of wheels?

The speed 900s, if bought at a normal RC shop- as they are normal, large RC motors are about £30 each.
Advantages. All stats are know, they are simple, rugged motors with build in radio interference suppression. And not that difficult to aquire, any decent RC shop can deliver them.
But their main disadvantage for heavyweights is the high RPM and low torque. @12V they run about 7000rpm and have 1.1Nm stall torque.
All this for 665 grams per motor.