If you have the money for it, I would suggest going with a service like ziptask or gun.io. They will do all the technical heavy lifting for you (figuring out requirements, vetting / hiring, project management, testing). You will likely spend around $15k-30k (very rough estimate) and have your app within 3-6 months. Then you will pay a couple hundred a month in server/bandwidth costs to keep the thing running.

Otherwise, you are looking for a "full-stack web developer". Though I would really suggest going with an agency for a project like this. Hiring and managing freelancers is hard.