Shimano says all of their drivetrains from 10 speed and up, and the brakes, are ebike rated. So i guess there is no need to go for Shimanos ebike specific components. They just seem to be overpriced.
10 speed seems to be the most cost effective drivetrain, and it also has enough range for...
You could reach out to the developer of emax. Apparently he wrote the program in the first place to solve this problem, so he knows a thing or two on this topic.
See post #11 in the link for further informasjon
Upgrade to Di2 shifting - EMTB Forums