If it will be run only on a flat track, a single motor would probably be adequate, but if any hills are involves you will need two.
With an all up weight of 250lbs, I think that the HBS hub or the Magic Pie would be the obvious motors to choose between.
However, if the track is relatively flat and the body of the vehicle is aerodynamic enough, the geared minimotor could be used to maintain an acceptable speed, and the current should be low enough to prevent motor damage. The minimotor would be able to freewheel with much less drag than either of the other two direct drive hubs on any slight downhill parts of the track, which could be a real bonus when economy is your main objective.
One problem that I can see straight away is the fact that you intend to use a 40 spoke rim, because the GM hubmotors are designed to be used with 36 spoke rims, therefore building the wheel is not going to be a straightforward job.
If it didn't have to be a four wheeled vehicle, a three wheeler would be a more obvious choice, especially if you're using very basic suspension, or even no suspension at all.
Alan
