looking for suggestions and advice

Any 'V' frame bike, preferably a 26" men's bike(no rear suspension) will be your best bet for a first build. Many of the cruiser style frames will require a little fabrication in the front motor mount area due to the 'S' shape of the down tube and sometimes oversized tubing. With that said, the cruisers with the fatter tires will give you a better ride than a thin tire road bike. Mountain bikes, especially the type with standard diameter tubing as opposed to the high end exotics are also a good choice. If you're interested in a used bike instead of starting with a new one, check your local Goodwill stores for great deals. Craig's List can also be a source but watch out for the scammers. Buy from a local seller only and don't send money to someone you don't know for an unseen bike.
Tom