It's always good to be prepared. However, I think you might be considering the worst. I'd go in and talk with them. Chances are they're going to either already know what to do, or be very happy to learn how to help you with everything.
There really isn't much to address with them. A letter is already 90%. They'll see you, then assign you an endo. At most they may have you briefly speak to a counselor (the larger hospitals have them on staff for walk ins), so that their therapist can review the information from your therapist and refer you to an endo from there. It may be a week or so for them to line up an Endo either on staff, or a specialist off site.
If you run into problems, Patient Advocate is the best one to talk to. They get things rolling when you feel put off.
I have a feeling that you will have a very good experience.