I've seen a couple of Schwinn Heavy Duty bikes on Craigslist lately for fairly cheap. They're old style, but not old enough to be worth anything as antiques. I think they're still being made, similar to the Worksman and other industrial bikes.

None of these bikes were intended to be motorized or ridden at 25 mph for hours on end, so you're on your own there.

"Lightweight" = new aluminum frame.
"3- speed" = old steel frame with thin 26x1-3/8" tires.