Hi @obstriker! Welcome to the forum! Your build looks pretty good so far! I have a similar build to yours! One thing I want to set on the table is that you are going to end up spending money on an enclosure, hardware, wires, and connectors. Think about $50 if you keep it simple.
As far as the motor and battery goes, https://longhairedboy.com/pages/esk8-resources this link will take you to a spot where you can find all sorts of useful info. I can tell you right off the bat that unless you have a weird gear ratio you are going to be looking at a pretty low top speed. My board uses a 260 kv motor (about the max you want to get) , a 6S1P lipo system, 15t and 36t motor to wheel pulleys and 90mm wheels. It goes top speed of 26 mph at full charge.
Best of luck! Building an electric long board is a truly rewarding experience!