I would use a midmotor. It looks like there is enough space in the triangle so that the motor wouldn't hang under the frame. It will also allow you to keep your gears.
If you want to keep the coaster brake and not switch to a freewheel hub you would need a tsdz2 which works with a coaster brake. The bafang is a freewheel motor (when you backpedal the chain ring doesn't turn). They were supposed to make a coaster brake compatible version, from what I read online, but I think they never did.
I see that many people think that with electric conversion you need a brake upgrade. Not really, the difference is not noticeable, plus you could always use a disk brake in the front - and the front brake is the one that has more stopping power.