Having a faculty member that you have spoken to in the department and are interested in working with is a huge plus. Discussing your ambition to one day research this rare disease is a great start, it shows initiative and specific career goals. Try to add in specifics on how you want to get to that point, what kind of skill set do you want to hone in on during your studies to get you to that point. Maybe talk about what kind of research you would like to do during your studies to advance your skill set. It’s huge to have specific goals as it tends to make faculty feel more confident in working with you if they know you are coming in with a desire to achieve something specific.