Setting expectations regarding hourly pay and research work schedule when busy with coursework
If this is the U.S., then you wouldn't talk about number of hours or pay scale in the proposal.  (I don't know about other countries.)  I imagine that what he has in mind is a brief description of a topic, or some topics, that you are interested in working on.

In my experience, professors are very understanding about the ebb and flow of the student's time commitments.  It could be helpful, however, to let the professor know in advance which weeks are the ones where you know you'll be tied up with exams, etc.