I would add the suggestion to be prepared for the editor to request a look at the manuscript (not just a short sample) to prepare an accurate estimate. This has been my practice for some time, and it allows me to provide a flat fee for the entire project. This makes budgeting much less complicated for the author, although of course I am willing to bill by the hour if preferred.
If neither Sally nor myself feels quite right for you, then I suggest you browse the bookstore or even the public library for books published in the last few years that remind you in some way of yours. Check the Acknowledgments page to see if they thank an editor. If not, see if there is any information to contact the author and ask if they used a freelance editor and if so, would they recommend them.
As a freelance editor, I would like to express my strong disagreement with the idea that a freelance editor must work as an employee of a publisher for three years to be any good. In my field, spirituality, many publishers have gone to having most editorial tasks beyond acquisitions performed by freelancers, so the in-house positions being held out here as a prerequisite for freelancing simply don’t exist; many editors must cut our teeth as freelancers. It also occurs to me that self-published authors usually bristle at the idea that they are not “real” authors unless their work is accepted by a commercial publishing house. Is it fair to apply this standard to the people they propose to employ as editors?
Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.