Think of yourself as a venture capitalist. Do you invest in the people with a good idea for a business, or the people already running a growing business?
If you can't get 100 people to join your free Ning site, why spend $50,000 hiring an agency to develop a professional community? That's madness. Ignore how successful you think the community will be. Focus on how successful the community is.
Don't invest in a community site before you have a community. The best time to build a website for your online community is when you have one, not before.
Every professional community site should be a sure-bet.