Hello and welcome to the forum. When you start your build you have to ask yourself a few questions before hand. . .
- whats my style (cruiser or Mt. bike)
- How will I ride ? (on or off road / both)
I would consider the Rockhopper first, it is more that likely a chromoly frame which is stronger then anything from walmart. Everything should be able to bolt right up to the rockhopper. The cruiser may require a little ingenuity, possibly having to use a large front motor mount and open up the rear sprocket hole to fit over the coaster brake. Nothing major, just speed bumps. Keep us informed!