Hello ratdad, welcome to the forum, glad you joined us.
I would go with the type of bike you like, if you like cruiser...build a cruiser. If you like Mt. Bikes then build a Mt. Bike.
Here is a basic frame dimensions that are needed for the Chinese 2-stroke in frame and a photo diagram below.
Most 24" or 26" bicycle with diamond frame and horizontal top tube should have sufficient space to mount motor.
At least 12.5 inches running along the seat tube from the top of the bottom bracket to the bottom of the top tube.Tubing size from 1-1.5inches on the both the seat and down tube. Plus or minus 70deg. above the bottom bracket between the down tube and the seat tube. Inner distance between pedals needs to be greater than 4.5 inches in width to clear the 4.5inch wide motor.
As far as the best motor, I'd just pick a vendor with great customer service.