Considering that you are going to trust an editor with your story, which you have worked on for so long, you want to make the right choice.
You should also make sure that the editor treats you fairly. Although hiring an editor is expensive, the rate should still be reasonable for what they offer. What exactly they offer should also be made very clear, before you pay any money. An editor shouldn’t demand any fee before they even decide to take on your editing project. Neither should there be any hidden fees.
A potential editor should be enthusiastic and preferably have experience in your genre. They do not have to outright praise your story – that’s not their job – but they should believe in the story’s potential.
Most important however is that you are on the same wavelength with your editor. You need to trust them with your manuscript and to be confident in the feedback they give you.
When you are looking for an editor personal recommendations of fellow writers are always useful. If you don’t have any writer friends who can help you, you can look up directories of editors or editors’ guilds in your country online and contact editors via those or paste your project as job there.
Before you decide on an editor, don’t be afraid to ask the potential editor about past projects they have worked on. Since hiring an editor is very expensive, don’t be afraid to ask all your questions, before making the editing project final.