I approve of this approach of taking a good bicycle and adding minimal e-assist. But I'd keep a 7 speed rear derailleur. And I'd probably prefer a rear geared hub to a front one.

We really need 36v-15AHr or 48v-10AHr and those bottle batteries don't quite manage this. Perhaps that could be packaged to fit in the bottom of the main frame triangle.

Obviously these guys are limiting power but the minimal geared hubs can be over-driven to about 750W. So 48v and 10-15A is both do-able and suitably peppy. Powered 25mph should be ok.