I've only had one small quilt finished by a professional longarmer. They were great to work with but $$ so it may be the last time.
She spent at least 45 min measuring my quilt, asking questions, making notes, etc. She made note of the quilting design I requested and we picked out variegated thread. I signed the notes and we both kept a copy. They had a backlog of 8 - 10 weeks before they could get to my quilt, so the notes were essential. I picked batting and backing fabric there, but they are ok with me bringing my own supplies. My LA does not have a large selection of batting either.
I would definitely get everything in writing, but especially if it is your first time working with someone. Its better to have no surprises at the end!