Logo design, creative concept design and creative mock-up of app functionality

I am hoping to develop an app but want minimal risk. As such, I need to mock up the look & feel of the app in advance of its build so as to be able to show it to potential buyers at minimum cost.

I start with a blank page, so logo, colours etc need to defined. Also I need a walk through of how the app would work so I can demonstrate it through PowerPoint.

The project is sensitive, so I would require an NDA to be signed in advance of payment.

This needs to be amazing so i am promising a bonus if the end result blows me away.