I'm close to that size and the only concessions I have made in donor mountain bike selection is to go with a quality model and a XL frame.
Think about it, there are 1,000's of 200 # plus riders (non ebike) out there who do 6 Ft., or higher drop offs, so you are not going to break a frame or chainstay street riding. I even do light trail riding, but I don't do jumps or log crawling, something my greared motors wouldn't like anyhow.
Even my hand built wheels are nothing special. Quality Alex rims laced with DT Swiss 12 ga. (bicycle size) spokes.
One thing I did do was fine tune my Fox rear shock w/ a kit supplied by Fox for heavy riders. It allows me to use a lower air pressure(for a better ride)without bottoming out.
Of much more importance is a frame that allows good placement of the battery and that is in the center of the frame.
And a donor bike certainly doesn't have to be new, in fact, it's a waste of money.
Yesterday's mountain bike technology is high zoot for a street Ebike and an extra few pounds matters little on a conversion where 20 Lb.s is added in the form of a motor and battery. Or in my case, motors and batteries.
There are number of threads here focused of "the best frame". Use search words; best, open frame, full suspension, etc.
In fact my 2003 Rocky Mountain Edge's 5" of suspension soaks up road irregularities with alpomb. In other words, mountain bikes the the serious riders consider obsolete, are still great Ebike donors.

Used mountain bikes are probably the best value in Ebiking. There are so many of them out there where the owner bought them with the intent to tear up the trails, only to find that trail riding is a lot of work, and they get parked.
Craigslist is your best friend.