Hi Rank,
This can be done, if you are willing to add some sweat equity. For example, I purchased a Dutch style Specialized bike for $100 and converted it with a 90nm cargo motor after refurbishing it. The good ones are not heavy, hunky/clunky. This one has an internal hub of gears. The motor pulls the heavy duty eChain through the shiftable gears. Because the battery is external it can be replaced with any brand of battery as the tech evolves and not only brand X at inflated costs for proprietary stuff that goes obsolete on purpose. With the matching rack and fenders it is 42 lbs. It tops out at about 27 mph and is a climber.