Welcome to the forum rockhopper, glad you joined us.
A good motor and the shift kit is not a bad choice. I have a Rockhopper with no shift kit and 30mph + is not a problem. You do know here in AZ we have a 20mph speed limit for motorized bicycles that are not registered as a moped.
Hope you decide soon on your set up, you get it going hope you can make it to Death Race