My friend had a REALLY bad experience becoming a professor's first grad student. Even when the research idea came from my friend, she did all the work, and she did all the writing, that professor claimed first authorship because according to the prof, supervising my friend takes up the time she could have used on her other projects. I think you should look at that professor's previous publications -- how many does he/she have in the past, how many are under review, were they first authors in those publications. I think that will give you a pretty good idea on how good the professor is, and how competent he/ she is. If she/ he has a lot of first author publications, I don't think she/ he is going to be so driven to claim first authorship on your work; and from my own experience, a competent confident professor is gonna want to teach their students, instead of feeling insecure and wanting to compete with the student.