I do have a question about the licensing -- will you be requiring copyright on code contributions to be assigned to you?
If not, you can't then sell those contributions outside of the terms of GPLv3, because you have to own them before you can relicense them, or obtain the owner's permission.
(If so, it's a little distasteful for the people contributing to know that their code is being used to make money for other people and they won't receive any of it.)