Hydraulic brakes - how do you plan to connect the brake cutouts?
I guess the battery options are downtube or a frog fitting? Any thoughts?
What size are your rims? 26"/700c rims are common, I was told 29er rims were same diameter as 700c but wider (but I've not confirmed)
Not relevant if you go for mid drive, but for info rear drive might be tricky - the alloy framed bikes I've looked at often have very shallow rear dropouts so making the axle & anti-rotation bits secure needs thinking about.
I've not tried any mid-drive conversions so I'll let others give suggestions about that area