^ Thoughts only. ..
I think the motor and power would be fine. The battery capacity is the same as mine, which is perfectly adequate (coupled to a 500w motor), but current offering are often equipped larger Ahs.
If you buy an ebike and like it you may get little use out of your current MB. So a conversion would be one less bike hanging around.
One thing I find wanting in my ebike is more gears (taller ones for on road). I'd look for a conversion that retains a multi ring crank set so I wouldn't be limited to the cassette gears only. But I think your MB is a 7 speed with single crank chain ring ? If so there would be no change to number of gears - though you might be able to change the crank ring size.
To your earlier post, I don't have a Rad but I'd say you'll want to be able to carry stuff because the e power makes it easier to do so. Run to the store, picnic etc.
I think either wheel size would work fine. Mine is 20x4 which is great for the beach and soaking up trail bumps but do over I'd go narrower, prob to a traditional MB width like 2.2. Lighter, less resistance and better suited to on road.
Two up on a bike ? Well, it's been a while ... maybe when I get my scooter