So I encourage people to write them, and it's a great habit to get into. Usually if you get called back, the firm considers you hirable on the basis of your credentials. You've already passed that test. So you want to be very, very sharp, primarily very, very enthusiastic, and you want to focus on, again, what I would consider the higher level questions, not the simplistic, sort of informational questions. Those are the questions you should be asking.
Negotiation comes after the offer.
You probably ought to do that research before the interview. They work best when you are single-minded in your pursuit of the job. Therefore, salary should not be part of that discussion. You need to do that research ahead of time. If the salary is not reasonable, then don't go to the interview.