Hi I would really like to hear more about the project, I have +4 years of experience as UX and web designer with Sketch and Invision being two of my main tools.
We can use Invision to build rough sketches using freehand, so that we can share those same screen with users for user testing. The magical number here is 5 users, that will show us around 75% of the issues with our firsts designs. For more detailed wireframes and high fidelity screens we can use Sketch plus Dropbox or any tool you have in mind for version control.
Also, if you plan to develop the app we could use a back end team to know your app endpoints, that will allow us to better organize the design features of the app on a product roadmap. For that we can use the 40/40/20 method for Lean design, organizing design features from MVPs or must-haves, Nice to haves and Innovation.
That would be my main process but we can talk details further, feel free to check out my portfolio and let me know any questions.