what would be the widest tyre you could fit to the 26" MP rim?
I have a 26 x 2.35 tyre fitted to the standard GM rim which is 61.5mm wide and it fits nicely, but you could have a problem with this width of tyre if you have mudguards fitted. The width of the frame where the tyre fits inside could be a limiting factor too.
On my bike, I find that the standard "V" brakes are more than adequate on the rear, considering they are in addition to the regenerative braking from the motor. I mainly use just my front brake since fitting the Magic Pie, because the regen is very powerful and the rear wheel can lock lock up very easily if I use both on a poor road surface. The regen on its own tends to works similar to anti lock braking on a car as it does not allow the wheel to completely lock up. Even if the tyre starts to skid on a slippery surface, the wheel will still continue to turn slowly, giving it a much better chance of finding grip again. This is particularly noticeable when braking on wet grass at speed!
A disc brake fitted on the front wheel would give better braking, but you would probably need to replace the front wheel for one with a hub that is suitable for bolting a brake disc onto.
Alan