Read carefully what DW & C58 say, and maybe look at endless sphere where both post. If you're set on using a hub conversion (rear is best unless you have an IGH, belt drive or some system that makes it infeasible), Check Craigslist for a steel MTB (should be a plethora in your area)), then choose a propulsion system. You need to decide how much power. Hill Topper and Leeds have good reputations for low power, Mac and others for more. Should you live in a hilly area, a mid-drive is a good consideration and can be installed on an aluminum or steel frame. Luna Cycle is an excellent source (I have three, one each for my wife and me and one for any guests). Also, read the tutorials at ebikesca so you know the differences in motors, then ask any specific questions that are not answered there.