Cruisers and 1990s MTBS lend themselves most easily to motorization. The geometry is more relaxed, clearances between parts more generous, so if you want to make the process go as smoothly as possible, pick one like that.
Steel frames are better than aluminum, because they'll handle the additional stress much better and are actually worth fixing should they ever break.
Don't use a suspension seat post on a bike you're going to pedal, your ankles, knees, and possibly back won't like it.