It may not be the cheapest way to go, but IMO, it doesn't cost much to get a nicer donor bike in the form of a mountain bike w/ full suspension and disc. brakes.
In general, used mountain bikes are going to be a great value. Many folks. buy them, try them out a few times, find out how much work they are and park them. Compound that with the trend of technology that is moving so fast, it has clogged Craigslist with '90's up to fairly recent mountain bikes that nobody wants. 26 inch wheels, which work so well w/ hub motors are considered de rigueur and a few ounces that matter little to Ebikers are a big deal to serious MX'ers. That which is obsolete to them are perfect for us.
I bought all three my donor bikes(a '03 and two '06's) for half of the MSRP and all three were "show room floor" condition having only been used 2 or 3 times.
I came to this past-time from the World of Japanese sport motorcycles and I like the Fox shocks, shinny linkages and other bits of Bling.
Yet still, many today promote the idea that suspension detracts from the "bicycling feel", to which I would ask, :how many ebike builds say true to the concept of "bicycle first"? At least for very long.
I would summise that thoses that give weight to the negatives of suspension , live where the roads are better that I see or maybe, just better riders. Try as I might, I'm forever hitting things I didn't see. Things that might not matter much @ 15 mph, but sting @ 25 mph. My full suspension bikes have not added much maintenance and years later, are still a joy to ride.
The fact that I love my ebikes so much probably have something to do with the rolling stock I selected to start wirth.