Honestly there is no right or wrong answer here. Its all about finding the right tech partner that you have confidence in. I have worked with developers that are great with UI/UX and designers that understand development. Either way find someone who has an understanding of the other side.

Personally I have both done UI/UX designs myself, whilst also managing teams of designers. The key is understanding the technology. The person who steps up first must have a clear understanding of mobile app industry standards, the features you want to build and a firm knowledge of how to represent those features in a design. Always look through their portfolios and find examples that are close to or inline with what you want to build.

The best course of action from my perspective is to get a software development company onboard that has specialists in both design and development. They should communicate and ensure that the UI/UX is inline with the tech.

For example my company, LaunchPadD, operates with a team of specialists from UI designers to native mobile app developers. Our team will support you even before we are engaged to ensure you are on the right track and not wasting your money.

You should be able to get any developer to go through your idea for free and give you a fixed price quote for design and dev. I would avoid having open ended hourly rates. Some will take more time and be more accurate than others so expect at least an hour or two of questioning for them to understand your idea even with the wireframes and then a day or two for them to come back with a figure (and breakdown).

If you need any further advice please feel free to get in touch.