Hi Mak,
Welcome to the GM forum!
I agree with dirthginge, go with the infineon. A 40 amp is a good match to the Magic Pie and it's very powerful, however that depends on the battery you can afford. Your battery will need to be capable of supplying 40 amps constant, depending on your terrain, if you live in hilly areas you will draw the 40 amps more than on flats. A 20 amp 48 volt ping battery should be suitable. I don't think GM's batteries can supply that kind of power, however GM's batteries will be fine with the Magic Pie with internal controller, but obviously with a lot less power than the external controller option! LiPo batteries are the only batteries capable of delivering very high currents from a small pack, but if you are new to all this E-Bike stuff I would suggest a Ping battery for your first setup as Lipo is not as simple as plug in to charge and walk away!
EDIT-------- OF course I forgot that you can get LiFeP04 to handle higher C discharge, But the point I should have made is the fact you can build a smaller pack from LiPo that can output far higher C than LiFeP04 for a lot less money! Though th einitial cost will be high because of the fact the charging equipment can cost a lot!
The thing you really need to decide is weather you want to use throttle all the time or just use it for boost. If you just want boost for hills etc then the pie will really put a strain on your legs with the motor drag. I would suggest trying the 250 watt geared motor which is small and light and will make a bike feel more like a bike, and most of all it will freewheel with no motor drag. I have no experience with that motor and can't tell you how powerful it is, but it won't take you 50 kph, 40 maybe! You will still have to put in a lot of effort on hills but it should take the sting out of them!
There are more powerful geared motors around if you look around, they can have issues with gears breaking, though that's mostly because some people use them at power levels that are way above the rated levels!
You can't use regen for braking with geared motors but with E-Biking I would suggest a bike with hydraulic brakes anyway!
The other thing especially if you go with the Magic Pie is you absolutely must use 2 torque arms if you don't you can get seriously hurt if the axel spins in the dropouts! The pie will shred dropouts on aluminum bikes!
One thing for sure there is nothing like the feeling of the first spin on an E-Bike!
Mark