I run a mobile development agency for the last 15 years. So I am a little skewed.

Before you start looking for somebody to get the app developed, start with very detailed specs. The more detailed it is the better. After that start creating the user flow. There are tools like invisionapp.com to create good app screen mockups. another decision that would impact is going going to be which platforms you want to create the app, native is, native Android or both or or a hybrid app like react native. In your case, a hybrid solution might be better if you want the app to support iPhones and Android users.

You get some high level estimate using some tools like https://www.app-estimator.armia.com/ . this would help to get an idea what to expect when you talk to developers. Get few proposals from freelancers and agencies.

Good luck with your app