Professional editors are for self-publishing, really. If you’re confident you’ve made the book the best it can be, query agents rather than paying for an editing service: a good agent will give you edits anyway, and they’ll be free (well, if they sell the book they’ll get 15%), and may prefer to work with material that hasn’t been tinkered with by someone already. Don’t waste your money! (I say this as a professional freelance editor.)
Three structural edits and a fourth polish edit (what I always call the “jokes!” edit – where your structure is finally sorted so now you can make the language sing) and a proof, sounds right. That’s what I did to get my agent, and have done on some subsequent novels - though some material I just send her early on, when it’s still shite.
When you can’t improve it any further, start querying: you’ll need a good cover letter and a synopsis. Make a list of agents – Twitter is good, find people who represent books you like – and query, say, 6 or 7 of them. Follow their query guidelines RELIGIOUSLY: my agent likes 10 pages pasted into the body of an email. Another likes 10k words on paper, like the 90s. Another likes first 3 chapters as an attachment. Etc.
Make a spreadsheet of all the agents, their requirements, and the date you queried. The reason to query 6 or 7 is if you get all form rejections: a form rejection being “no”, without any personalised note of encouragement or feedback. They’re not inviting you to query again so even if you rejig the book, they won’t open your query. But you should have other agents on your list, so you can rejig the book, send out to the next 6 or 7, and so on. Same goes if your first 6 agents all give you similar feedback but don’t want to see the book again: take the feedback, edit the book, send out to the next 6. Etc.
You might not have to do this – I struck gold on my first batch of querying and have been with the same agent ever since.