The single most decisive factor should be the quality of the edit itself: Will this editor help improve your book? In order to assess this, nothing beats a sample edit (even if you have to pay for it). Testimonials and credentials (such as experience at a major NY publisher) can help you narrow your list, but you should submit the same sample to a variety of editors and compare their work side by side. Some editors will push too hard, and some won’t push hard enough; some will simply “get” your writing, while others will seem to speak another language entirely. In my experience, the editor’s résumé is far less revealing than the quality of the sample edit. In fact, the higher an editor’s pedigree, the more reluctant the editor might be to provide a sample edit. Don’t be afraid to insist: no one should expect you to invest in a car without a test drive!
Specialty – Do you want a copy editor who will strictly follow the rules in the Chicago Manual of Style? Do you want someone whose specialty is line editing and can write smooth, clear, and creative sentences? Are you looking for someone to make sure you don’t have any glaring grammar or spelling mistakes so as to avoid the brutal grammar police reviews? Or do you want someone to look at the overall structure or plot of your book?
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.
In fact, Pressfield writes, “Resistance is experienced as fear; the degree of fear equates to the strength of Resistance. Therefore the more fear we feel about a specific enterprise, the more certain we can be that that enterprise is important to us and to the growth of our soul. That’s why we feel so much Resistance. If it meant nothing to us, there’d be no Resistance.”