Has anyone hired a dev to create an app?

I am a software engineer myself, but time > money so I prefer to hire a qualified person to build my app instead of doing it myself.

I just have a few questions on the off chance that someone has already had some experience with this:

1. How did you find the developer (elance/upwork) ?

2. How much did it cost you?

3. What were the biggest challenges?

4. How long did it take to develop the app?

5. What physical information did you provide to the developer when explaining your concept? (mockups etc...)

6. How did you do the payment? (deliverables?)

I am sure I will have more questions, will post/edit when I think of them.