I think it is safe to have motors as long as there is a speed and torque cap. Maybe, you can have two modes programmed, a "high" speed one for long-distance travel outside, where no one is, and a "slow" speed for indoors.
Also, for the controls, maybe you can place pressure sensors on a handlebar so that the motors will only trigger if it thinks your are trying hard to push the cart. I think that a Window motor will just be perfect because it is high torque, low speed. Also, make sure it isn't too slow, as that would render it as more-or-less useless because it would take to long to transport the cart in this way. I think it would be wise to stay away from CIMs because they are high speed low torque so you would need to add money and weight to gear them down. Also, if there are motors, you should probably have a physical "emergency" switch to break the circuit if the cart goes out of control. Could save a few lives if the cart is heavy
