Asking professors’ help on proposal
What you propose seems fine to me. Make sure, however, that the professor understands why you are asking and that his/her help doesn't become authorship. Academics work together, especially at the idea stage. 

Some departments provide a coffee room with a table and whiteboard just to encourage this sort of collaboration. 

Note that the answer might be different if you had no ideas at all about where to start. But the prof can, as a minimum point you to places to help you develop the proposal. I think that most professors would consider this to be fine, or would tell you (gently) if they didn't. Ask.