If you are mechanically inclined and have done some reading on newbie tips and China engine build tips, then by all means go all out for a shifter bike and don't waste your time on a single speed motorized bike.
This hobby isn't rocket science which requires you to start out on a Cessna 150 before you can advance to flying the space shuttle.
Once caveat though, if you don't which end of a screw driver to pick up, do yourself a favor and buy a moped.
As for kits, I would only recommend Dax with Pirate's PK80 being a close runner up. Both vendors have an outstanding reputation for customer service and decent products. My current and only ride has a Dax engine that I've abused for the past 4 years on many mountain rides in the summer...going slow, running very hot at high throttle settings on continuous mountains grades has sold me on Dax's engine's (and using a castor oil blended oil mixture).
The only shifter kit I can recommend is the original inventor: Sick Bike Parts and make sure you spend the extra money and get the heavy duty freewheel up grade.
Good luck and if you need any helps or tips, IM or email me with your phone number and I'll gladly walk you thru the build process and show you what I've done on my bike.