Hi Sonavi,

Getting back to your question about diagrams to use. I see you mentioned you have a User Interface as well as the database. 

I find it useful when working on UI's to create a site map using a UML state diagram. The state is a screen and the transitions are the actions taken to move between the screens. Simple to knock up and useful for understanding. Users like it too.

Good luck with your project. Let us know how you get on.

Kimbo