My guess is some combination of:
- Money
- Initial pre-committed residents
- Habitable land
- Management
And then the main thing left, is, I believe, approval?
Which leads to my questions:
- What is the legality? Where is it legal? Where is it illegal? Or if legal, what are the chances/what is needed for approval?
Establish a development committee and some staff to bringing commercial businesses and people looking for office space (so the people living there have some place to work). Local county/city transport authorities to build transit links into the development. Ensure they can get into and out of the area cleanly, it won't work if it's an island. Most of your population will be commuting into and out of it no matter what you do.
Build it in phases and sell it as you go. You'll need to build the residential sections first (businesses can't survive without customers).
Hire an advertising team to draw people in and blanket the region with adverts. Work with local real-estate brokers to hi-light this as a destination they should show all clients during a house hunt -- even people not looking for a place like this.