If I may, I'd also like to add: Do research into the editor!
See how many books they've worked on that actually get put on the marketplace or publishing platform. Read the work they've worked on if you're able! If you can spot mistakes and the work just isn't pleasant to read overall, that's a sign to run lol.
Don't be afraid to ask for a sample edit! Some editors offer to do a sample edit of a set amount of words from your work for free or a small price. This way you can appraise the quality of work they do and if you gel with their style.
As for rates, from my research looking into various professional freelancers, about 2 cents per word is the ballpark rate for line edits. So if you see someone offering less, definitely look into them more; they could just be starting out and still do good work or they could be not-that-great. Of course, a person could be overcharging as well, but it just goes back to the research you ought to do in the first place.
I've personally thought of offering editing services as well for prose, but I don't know if I'm good enough yet. Like I believe I have a good grasp of things to look out for: dangling participles, sentence structure, the differences between 'into' and 'in to', all the tenses of lay and lie, when to use 'nor', and other things I've researched when writing my own work. I also know I can definitely help with developmental editing since I enjoy analyzing stories and films.
Perhaps once I've actually gone through the process of working with an editor of my own, I would have a better gauge.