Should I unit test Repository on it's own?
I don't think there's much point. A Repository is supposed to act as a layer of abstraction between your business logic and the database, so by the time you mock the database for testing purposes, all that's left to test is the Repository's API endpoints (a test that I would regard as "uninteresting", and already covered by integration tests anyway).
Will it be enough if I mock IRepository and only test UnitOfWork?
Of course, depending on what you consider "enough." This is the appropriate place to mock a business operation, in my opinion, since the expected values are those required of your repository mock, and not those of a database (which would require an integration test).
To put it another way, your repository is already expected to return the "correct" value. If you're not sure about that, then include some integration tests for your repository (wired up with the proper data-source) to make sure.