My middle is using mobility scooter motors and gearboxes, although not finished yet, from early tests i have concluded the following.
1) very easy to do
2) can be picked up on e-bay from about £20 upwards.
3) bit slow as gear ration on my ones is high (worm drive) so high torque low speed.
4) to counter this either use bigger wheels or increase voltage. Currently running my 24v ones at 30v and not getting even warm. ( although not a true test yet. ).

Hope this helps