I've been thinking about these:
They are 2kW but at 193k and 37V they are 7000 RPM which is more easily geared down. $52 each with discount. But it's an outrunner. I had an idea to use six of these mounted on the axle and use 12 tooth 20 pitch pinion gears on each shaft ($12 ea). Then use a gear rack on the inside of the brake drum which is probably about 12" diameter and would have 240 teeth, cost of about $35. A This would be a reduction drive of 20:1 so the wheel would turn at 350 RPM. So using this method a 12kW wheel motor could be constructed for about $419 in materials.
It would not be too difficult to have 24 tooth and 36 tooth pinion gears on the motors and rig up a system that would shift gears by moving the motors radially and axially. These gears would add 6*$21 or $126 to get the second speed of 700 RPM and then 6*$31 or $186 to get a third speed of 1150 RPM.
This would not be a 15 minute bolt-on accessory but could be fairly easily made to replace the brake plate on the rear axle and the brake drum could be machined with the ring gear.
For caliper disk brakes you could mount the motors with the shafts on the outside of the disk, and you could just cut gear teeth on the outer circumference of the rotor. A simple machine shop operation and you retain the use of the brakes. The caliper may limit the number of motors, but you might be able to get larger ones or make do with three or four and lower power.
Here is my design for a wheel motor that I could build for a tractor application. These are three 1/4 HP (1/2 HP peak) 12VDC motors that I have. I'm showing three 12 tooth 20 pitch gears and one 60 tooth gear that is also the hub for the wheel. I'm showing an 8" pneumatic tire and wheel assembly from Harbor Freight, which has four bolts about as shown. The motors are mounted front and back on swivel arms so the pinion gears can swing out of the way. This is only a 5:1 reduction and my motors are 5000 RPM, so this will spin too fast and not have enough torque for a tractor. But this is just a concept.