Start testing already existing Django project

I just read the whole Django documentation about testing and I need soon to start testing an existing project. The project, a dashboard, is pretty big and I was not involved in the development phase (yeah, it sucks). There will be unit tests and integration tests (with Selenium).
Here are few questions:

1. Is there any suggestion to start in the best way to test an existing project?
2. Which framework / tools should I use? (actually using only 'coverage' and 'mommy_model')
3. Should I start from the unit tests? Should I test the models first?

I feel very disoriented because it's a whole new world for me, but I'm really exited at the same time. Hope some experienced tester can show me the right path.