Your build is close to my heart. I actually begged Dave for a 100MM adapter years ago and built the first Tangent powered (3KW) fat bike. These days I'm running a 6KW Tangent All mountain setup. I've tried all kinds of gearing combinations with these bikes, and have a pretty good feel what brakes work with what.
First order of business is going to be what chainrings to use. Your inner chainring that goes to the motor is going to need to be the smaller 24T most likely so it won't rub the chainstay.
I hate these chainrings personally because they are weak, it's easy to bend the teeth, and the chain tends to pop up on the teeth. If you can run a 30T 104BCD chainring on the inside with enough clearance to the chainstay I'd recommend it.
If it's too close then you need to go to the 24T, keep in mind the Tangent has enough snot to bend a cro-moly steel frame to the right and that chainring will saw your chainstay.
As far as rear gears I'd recommend starting with a cro-moly or at least steel freehub body. You'll want a cassette where the cogs are joined together and not individual cogs. Individual cogs will apply all the torque from the motor into a single spot on the freehub body and indent/eat away at the freehub body.
For brakes I'd recommend 203MM rotors front and back, and 4 piston brakes. The additional rotating mass of the wheels and tires makes everything harder to stop.
The Shimano ZEE's and XT 203 rotors would stop this bike in a hurry. The only issue was that with QR skewers in the front instead of a more modern thru axle setup, there was so much braking force it would pull the axle out of the right side drop out and cause the disc to rub the pads until I stopped to fix it. I have the same brakes on my current bike with thru axles and this isn't a problem.
I should prob show a pic of the bike huh?