What's the best set up designed to carry 300-400+lbs?

I am planning on towing a child carrier to carry the treasures that I find at yard sales, hence my requirement for a system that can handle stress and provide the torque required. I am trying to determine the best wheel size, hub motor, controller, and battery configurations to accomplish this task without over heating the system.
I like the idea having putting the motor on a bent or Down Hill mountain bike. It seems to me that Crystallyte and BMC have the better motors. I probably going to start the first build on a Trek road bike or 24 bmx cruiser, since they are paid for. This is my first build, so any suggested set up would be greatly appreciated.