Sounds like a pretty cool project.
If you go with a water wheel, make it as light weight as possible. Over or Under would work, but I think Over would utilize your water resources a bit more efficiently, but at what cost?. At what point do you release the water from the wheel? When it is no longer useful for the desired output, and this would be Torque. You are not trying to store energy in the wheel for an up and coming event, like a flywheel on a punch press. The heavier it is, the more your water has to work to keep it in motion making it less efficient. If you go with gearing on the output of the water wheel to produce the desired speed (HP) to run your pump, go with spur gears in a light oil bath. I would also enclose the wheel from the elements. But that is just me!
Good luck on your project! I hope to see pictures when available.
