If you want to write a book, write the book. Don’t build an audience. Don’t look for an agent. Don’t write a proposal. Don’t look for a more prominent person to ghostwrite for. Write the book.
When you can’t do the real thing—you’re not well-trained or qualified enough—get as close to the real thing as possible. If you want to become a doctor, it helps to spend a day shadowing a doctor first and learning what their job is like.