Electric wheelbarrow #2

Hello.
I finished the hub motor wheelbarrow that I talk about here 6 or 7 weeks ago and posted a few pics of....it seems to work great with no problems discovered yet but it will not be field tested until spring time under extreme conditions such as hot temperatures, rough terrain, water and moist conditions, long hours of use, so on.
I have concerns that because I built a water tight battery /control box case under the belly, it will have to much heat build up with no ventilation.
I'm also skeptical that although the hub motor seems to have plenty of torque, it will be stressed to its max when put to work in outdoor conditions.
And it has NO reverse!!!!!

I want to have a second prototype ready to be tested when my business starts up again in spring.
This unit I want to use a reversible, NON hub motor (preferably only 12 or 24 volts) that I can couple to a small gear box so that I can generate the high torque needed for a wheelbarrow full of concrete going up a hill in hot temperatures over rough terrain without the motor drawing its maximum amps to do it!......and still be able to switch to a higher gear for faster speeds while empty.

The help and advice I need is: (and greatly appreciated)

-The motor?????
Most golf cart motors are to big for a geared down, light weight wheelbarrow and they have electronic controllers (like my hub motor) that I was hoping to do without as I will be controlling the speeds and direction with a gearbox. I am not completely against using a controller as long as they prove to be reliable under sealed, hot conditions....remember, these wheelbarrows get bumped around and hosed down with water on a regular bases, not a good thing for electronic devises!
I think most golf cart motors also have NO shaft to hook up to, just a spline.
I was thinking to just use a simple solenoid on/off motor to run at full speed after the appropriate gear was engaged.
And golf carts and there components are not cheap to buy...even an old used cart that works ok is to expensive to buy when I will be throwing most of it away and just using the motor and controller probably. Since the batteries in old carts are usually gone or very close to it.

Any ideas what is my best motor option?? should I use a controller?? I hopefully want a motor that I can use less expensive 12volt motor cycle batteries.
I live in Barrie, Ontario, Canada....very little suppliers of this stuff here. Any advise on reliable companies to order these motors and parts from?
Thanks in advance for any help and advice.
Terry.