Mechanics and Fixing Any Questions Answered

Shifters (this'll be one of the thumbshifting ones)

https://www.sigmasports.com/item/Shimano/SW-R600-Climbing-Shifter-Switch-For-Drop-Bar-Right-Hand/3B97?

For shifting you can also use XT Di2

Junction box (any particular one, or are they all the same)

https://www.bikeparts.co.uk/products/shimano-sm-ew90-a-e-tube-di2-junction-a-3-port?

If you want to be able to see the gear you are in you can use the Alfine or XT/XTR Di2 one.

SD50 cables

1 short cable from shifter to junctionbox at stem/handlebar
1 long cable from junction box A (stem) to junction box B (https://www.sigmasports.com/item/Shimano/SM-JC41-Di2-Internal-Cable-Junction/WI9)
1 cable from junction box B to battery (length depends on where the batt will go)
1 cable from junction box B to alfine motor unit

Battery (what's the difference between this and the external battery?)

This is the external battery: https://www.probikekit.co.uk/cycling-electronic-components/shimano-di2-sm-btr1-battery/10781871.html?

It comes in 2 or 3 mount options depending where you want to mount on the frame; https://www.wiggle.co.uk/shimano-bm-dn100-battery-mount?