Your planned publishing path is very important to the decisions about what kind of editing to get and how much to pay for them. Some people will tell you not to pay for an editor if you plan to traditionally publish; after all, the publisher will hire editors to make the changes they want. But that only happens after your book is signed. And getting signed is not always an easy process. So you may find an editor helpful to get your book ready for submission to agents and publishers.
Here are the factors I recommend considering before hiring an editor on a manuscript slated for submission:
Are you within the right range for the word count?
Agents and editors will reject a manuscript for word count alone, so missing this important checkbox may severely hinder your publishing chances. An editor can advise you on ways to adjust your word count. But you may be paying extra for an editor to read scenes and chapters that you'll cut, so you'll want to get as close to the word count as you can on your own first.
Are you going to be submitting to an agent or a small press?
Small presses tend to be less picky on their acquisitions and they usually offer less money to authors. If you are considering a small press, you may not want to spend time and money on your own editor.
Are you familiar enough with the industry to know your genre and age range and meet the agent's expectations?
Editors who know the industry can help you fit the latest genre and age range guidelines, as well as hit checkboxes that will give your manuscript an advantage during querying. If you don't know these things, you may want to check for editors who have worked in the industry for a long time or have connections to the traditional publishing world so that they can help you see how the industry will view your manuscript.
What are the weaknesses in your manuscript?
For authors planning on traditional publishing, I don't recommend paying for copy editing. Agents and editors don't expect manuscripts to be cleaner than spell check and a beta reader can handle. So don't bother paying for copyediting unless you plan to self-publish.
Agents and editors are looking for manuscripts with relatively minor plot problems. Editing a manuscript that needs major changes will often end up with a fundamentally different manuscript, so agents and editors have to be careful about taking these projects on without being really sure that the author is on board with their final vision. Because this can be hard to determine, many agents and editors pass on manuscripts with major plot issues or offer R&Rs (revise and resubmit). Cleaning up these issues with a developmental editor before querying can help put your best foot forward.
Gatekeepers also tend to look for generally clear and compelling writing. Different agents and editors prefer different styles and voices, but they all want a manuscript that is mostly already in that style. Editing every single line in the whole book would be massively time consuming. Instead, they look for manuscripts that need occasional tweaks but already have the basic style and voice solid. In this case you want to be careful not to have an editor change the style too far from what you personally are capable of writing. But you may find it helpful to hire an editor who points out issues or identifies reoccurring issues for you to fix on your own and learn more about. A first chapter critique, used correctly, can help you with this.
So before submitting and querying, you may find yourself benefiting from working with a developmental editor and maybe a substantive editor.
If answering these questions led you to decide you don't want to hire an editor, you may want to check out my blog posts on submitting and querying.
If you do want to find an editor now, check out my post about choosing an editor.
And if you haven't already, make sure to read my post about getting ready for an editor so you can get the best use out of your money.