Disable All-Terrain buggy, DC motor selection

[bertie991]

I am involved in a university project to design and produce an all terrain wheel chair. I have been charged with selecting drive motors but as a mechanical engineer i am unsure where to start. I have done some basic calculations to determine forces and power required however my results do not seem to tally with the motors that are being used by existing models.

What i know;
Approx loaded weight - 200-250kg
Required speed- 4 mph
Motor Output- approx 100RPM (using a 14inch wheel)
Required to climb gradients approx 15deg

How would i use this information to determine what spec of motor would be required for the buggy?

Thanks