I believe this is the first wheelbarrow that we have helped to add electric power to on this forum however we have helped people with other similar projects.
We should start with figuring out the gear ratio and sprockets to use to get the desired top speed. I imagine that you want the top speed to be walking speed which is around 3 MPH? So I can start working on the gear ratio could you please let me know the diameter of the wheelbarrow's tire?
This 24 Volt 350 Watt motor has just under a half horsepower and when geared for 3.24 miles per hour will be able to haul a 300 pound load up most hills and inclines, however if you need it to go up very steep hills or inclines, or will sometimes use the wheelbarrow to haul heavier loads, then you might want to consider using our 600 Watt motor. Here is a gear ratio calculation using our 600 Watt motor along with a 60 tooth wheel sprocket.
The 600 Watt kit will cost more than the 350 Watt kit because all of its parts are bigger and it has three batteries instead of two. However it might be worth it to go with the bigger kit so it can handle anything that you might throw at it.
Once you decide which size motor to use then please let me know how many minutes or hours per day of run time that the wheelbarrow will need to have, and then I can start working on what size batteries it will need in order to run for that amount of time.
Please let us know if you have any questions.