Mayo Clinic takes letters of interest and intent very seriously. With that being said, they want to make sure that you've thought about all of your options and that you think that this school would be the best fit for you. It should not be sent prior to your interview. In fact, I would wait several weeks to send out the letter. I wouldn't worry too much about the length of the letter but do not make it overwhelmingly long to read as the staff is very busy. Include why you like the school and how you think that it is the perfect fit for you (they like when your incorporate your values with mayo). As far as updates, I would only contact them if the update would some how help your application (new medical experience, research/publication, etc). If you have any specific questions, feel free to message me.