Project description is not self explainable but coming from listed states, I could say my approach:
1) Developing this application in extensible flexible way using MVVM and Dependency injection design patterns. This helps code to be clean enough, in case of other developer induct in the project would be much easier for him.
2) Separating roles and decoupling functionalities.
3) Targeting it to all device families - means Desktop, mobile, xbox etc.
4) Project itself loos not so huge, but still there will be need of 20-25 days.
5) It can be covered by tests.
6) After finishing project I can support maintenance of it.
P.S I am windows phone developer working on software service company with experience of developing windows phone app over two years and now windows 10 ecosystem applications.