Hire some people to start building it, publish the code under a copyleft license such as the GPL and start hiring consultants to contribute to it. This will give you control over the critical "must haves" while making it possible to eventually spin a lot of the maintenance off to third party companies.
Software that's developed this way has a long history of being very high quality as there's much more cohesion and communication of theory when the developers and costumers work for the same company while at the same time the GPL will protect the project from potential takeovers via internal politics.
Hire some people to start building it, publish the code under a copyleft license such as the GPL and start hiring consultants to contribute to it. This will give you control over the critical "must haves" while making it possible to eventually spin a lot of the maintenance off to third party companies.
Software that's developed this way has a long history of being very high quality as there's much more cohesion and communication of theory when the developers and costumers work for the same company while at the same time the GPL will protect the project from potential takeovers via internal politics.