1st off, welcome to the best place to get information about you motorized bicycle on the web.
There is nothing like building something just the way you want it and being able to say "I built that". If your preference is to have your bike built for you, then there are tons of links here in the forum that will guide you in the right direction; plenty of people to ask and see what works and what doesn't. Be sure to use the SEARCH feature before posting a new topic, as the topic may have been covered one or more times already. As far as a pre-built motorized bicycle I have read good things about Pirate Cycles and Spookytooth. Keep us posted.