Ahh well thank you very much, I appreciate it!
Well for a mountain board you may as well go for two motors, you get more power and torque and you would have plenty of space for all the electronics. The turnigy SK3 213KV which I have has been great so far, no problems with it. Two of those motors should get you going! Also if you would kindly subscribe to my YouTube channel as I will be doing a mountainboard build in the near future... :) https://www.youtube.com/user/PracticalProjects
I'm not sure i'm afraid, depends on what esc and receiver you have, I had problems with that and I just swapped out the entire receiver and controller
Yeah the wheels are 70mm, I calculated it a different way to you, it's definitely quicker than 10mph I'm sure