I implemented this way of working years ago and see how empowering it is for authors, and helpful for editors to boot. This doesn’t mean you’re asking your editor to do a free sample edit. (I discourage editors from doing this and caution authors who push for it as asking editors to work for free before you hire them may result in resentment.) Paying an editor for a few hours of their time to ascertain whether they’re a good fit for you is a great way to put your toe in the pond before submerging yourself. You can then see for yourself if the editor’s suggestions and comments resonate before moving forward with the whole project.

OUR EDITORS are confident, calculated, experienced and ruthless. We tend to look for people with at least an MA in Literature and 2 years of full-time book editing experience, but will also give editors a chance if they’ve edited at least 10 books and can provide references from happy clients; or if they’re successful authors themselves who have published several books.
In return for the sacrifices you must make in order to self-publish, you will gain total control over the project. In addition, any profits from sales of the book will be yours (not just a royalty percentage as with either a traditional publisher or a vanity press), and in some cases it may be the only way of getting your book into the hands of readers. However, there is no getting around the requirement to get the tasks done one way or another.
All points well taken, Derek. (To be fair, Teymour does address the different levels of editing.) I’d add that, as far as the surgeon analogy, even the best editor can’t fix a mediocre story. Some stories are dead on arrival and no amount of resuscitation is going to help. Also, an editor might be able to fix some things, but the “patient” also has a lot of work to do after the operation is done.
Another great resource is Reedsy; a platform for finding freelance editors, writers, and all things book related. You can also use the Editorial Freelancers Association to help narrow your search. These websites are fantastic because they vet their members, meaning you will only be viewing the best of the best when it comes to editing. Keep in mind, Reedsy prices will typically be higher than going to an editor directly because the website, rightly so, takes a commission (from both the editor and the author) for finding, vetting, and then managing your contract.
While you may (or may not) be correct in your assumptions, your tone in general is rude, and disrespectful. If you are indeed a true editor of distinct taste and experience, you should not feel the need to bad mouth others for their thoughts and opinions. Show a little decency, and realize that you are indeed speaking to a human being, who probably has thoughts and feelings, and is in my opinion, a much more intelligent human for having the intellect to not attack someone verbally for their thoughts. No one needs you to be rude, or asked you for that matter.

Book editing is both subjective and not. There are rules, and yet the rules of writing are often intentionally broken. You want to find someone who knows the rules, who will fix the writing, and who’s flexible enough to know that voice is sacrosanct, and that editing out colloquialisms, slang, and humor is a no-no (unless it’s overdone or not working, that is).
Start with a google search and see what comes up. Click on a handful of options and see if you can find an editor who is specialized or who has edited books similar to your own. An easy way to do this is to open just a few of your favorites in tabs and check out each editors’ website and about me section. Take a look at their testimonials. See if any stand out to you.

Regarding your last question, I know of no editor who gives a money-back guarantee for occasional errors that happen to slip through or for a different opinion by a later editor. I’m afraid such things are just the nature of the publishing beast. But by having a shorter portion edited first, you will get a decent feel for whether someone is the right editor for you, and will be able to be confident in his or her work on your entire book.