Re: So i am thinking about building one.
Like seinfeld said in that movie, "Do It!" :-)
Leave the coaster brake on for starters. Just be aware it's not effective at the speeds a motorized bicycle is capable of. ...but you were going to start out slow, right? :-)
After you get it running you can then make plans for additional braking after reviewing the brake threads here on the board. There's SO MUCH good information!