as for the wheel strength enverlope, I get your point, A freind who is a real bike nut was sugesting using 40 spoke tandem wheels? to try to get round this problem.
The Woosh Big Bear may be too big for you at 5' 7" - others on here have and have ridden them so they might be able to expand on that.
Forty spoke wheels seems overkill unless you are closer to 30 stone than 20.
A reasonable quality 36 or even 32 spoke wheel should do the job.
Ebikes are the same as ordinary bikes in that the more you pay, the better chance you have of getting quality components.
A crank drive bike allows the fitting of branded wheels, so you know what you are getting.
I ask a fair bit of my wheels weight-wise and have had no problems with the Rose in my avatar - Mavics, and the AVE - Alex.
The Kalkhoff you've looked at ought to have similar quality wheels which will do the job.
Whatever you buy, check the frame size and wheel size carefully.
I reckon you are a bit too short for 700c/29er wheels, so look for something with 26".