Hi - for general tips, please read Section A and B of the
Complete Competency Interview Guide I wrote last year. It covers in depth my preparation method, the most commonly met questions, and the most important advice for succeeding in VIs.
For the second question, I think you can safely go back 2-3 years for an average deal the firm on to still be considered relevant, and potentially even 5-6 years if you will look at deals/cases of historical importance and/or you are making a broader point about the firm's historical excellence in the area or its long-standing client relationships.
For the third question, I expect you will be asked at most one direct question about a the firm's cases/deals and client base. As such, I think one of each should generally suffice. Nonetheless, if you plan to use examples of the firm's clients and work in other answers (such as why the firm), you are of course allowed to do so as often as you want.