A copy editor or line editor focuses on improving sentences and grammar, as well as suggesting other ways of rephrasing ideas, and providing comment on anything else they pick up. Non-fiction books often use multiple styles, for example, chapter headings, sub-headings, action points, bullet points, call-outs and examples, so these will be examined at this phase.
The job board is NOT a big reason to join the Den, especially if you’re a new writer. It’s a higher-end board that doesn’t post any low-priced gigs, and is a good resource for mid-career writers. But in general, the board is a real sidelight to the point of joining the Den, which is to access 300 hours of training+, 24/7 forums for support, live events where you can ask questions of experts, and more.
Some authors budget very unrealistically, perhaps just a couple hundred dollars. (I’m not saying this is necessarily true in your case, but I have encountered it.) In such cases, I find myself wondering whom they are imagining should bridge the gap between what they are comfortable paying and what the editor needs to earn in order to pay expenses and support him- or herself.
I cannot speak for all editors, of course, but I know I and many of us would be open to editing just part of your book at first, say, a particular chapter or two that you sense needs a lot of work. That is often enough to show you mechanical errors that you tend to make repeatedly. You can then go through the entire manuscript yourself looking for as many of the same kind of errors as possible. You will sharpen your basic writing skills in the process, and it will pay off in all future projects.