I converted a Trek 830 mtb bike 5 years ago, Steel frame made around 1991. It was not difficult. Rear hub motor, about $200. Battery on frame, about $280. I found the center pull brakes were too old with corroded cables. The bike had posts for cantilever brakes so I swapped. Still use the original derailleur and the eliptical crank. Put on Shimano trigger shifters. Still have the origiak seat. It was my son's bike in High School.

It's only a three level pedal assist with throttle. Not too fast. Tops out around 20 mph. The Trek was always heavy, Motor/battery bring it up to 52 pounds. Still lighter than your average store bought ebike. I like it for my senior riding. I have since converted a few others, which are lighter, but this was my first.