Hi EvoX,
I have been designing App UI for my current company and as a freelancer. I am a graphic artist, illustrator and UI designer.
Flat Design wouldn't be as problem as I have used it once for one of my client's app design. I am thinking we can also use flat stylized icons and a good color scheme for your app. Although I will be needing more information about your app (i.e functions, pages, etc) Also will it be for android or iOS.
I hope we can discuss more about this project.
Thank you,
Best,
Czarlene