This conversion is not quite as straightforward as it might at first appear.
If you are planning to use a geared motor, you are going to need reverse as well as forward speed control.
This could easily be accomplished by using a simple 500 Watt brushed motor speed controller combined with a thumb throttle for speed control, and a double pole double throw switch (or a push button operated DPDT relay) on the two motor leads to change direction.
Another problem will be making sure the outer diameter of the tyre is correct for the wheel barrow frame being used.
If the diameter is too small, the rear legs will be too close to the ground, making it very awkward to use, as you would need to bend your arms considerably to lift the handles high enough to give adequate ground clearance.
If the wheel is too large, you would end up stooping forward to reach the handles, which would not be very good for your back.
The HUB-24M has an electrically released brake which could be controlled by a simple brake lever switch on the left hand handle so the brake would automatically release as soon as you grabbed the handles and could not be re-applied until you release the lever.
If you were to lose your grip, and the handle accidentally slipped out of your hand, the safety brake would be instantly applied.
Alan