Haven't read all 50+ answers, but here are my opinions:
Open-source the concept only. There are still Java, Ruby, etc. developers out there, who don't C#
Not much, as you would decline almost everything.
Don't know.
No, just keep the quality bar high and use modern scientific methods to optimize the community's behavior.
If you want to sell the engine, no problem there.
Btw, how do you imagine a business version of SO? A corporate knowledge base or something?
If I were you, and I was low on money, I would ask for donations, and perhaps ask god to help me out some way. Or any minor gods out there.