As far as the profit margin, one barrier to just building this yourself is the NRE for a suitable brushless motor controller that does speed control, handles unknown and varying load torque, and does motor braking. Or do you know of an existing alternative controller?
They are using off-the-shelf brushless motor controllers (from what I can gather) and off-the-shelf brushless motors. There are many makers of ESCs with your requirements including from the now-ubiquitous HobbyKing, both sensored and sensorless (for RC car applications) including this one:
http://www.hobbyking.com/hobbyking/store/__17234__Hobby_King... which will do variable engine braking as well.
Our motor control ESCs, remote control electronics, battery management systems, and software are entirely custom. Off-the-shelf RC parts aren't smooth, reliable, safe, or quiet enough.
JohnnyCowboy's concern about early motor failure from the sideloading--do you have estimates on the mean mileage between component failures, and how much replacements would cost?
