A common question writers ask is, should they hire a freelance editor to go over their novel before submitting their work? After all, these folks are expensive. Are they worth it?
Well, here's what you need to do: if you can afford an editor, can you afford ten or twenty times their fee? Because here's what is going to happen: your agent will likely ask for rewrites before submitting your work to editors, and editors absolutely will ask for rewrites, many many rewrites, before publication. Are you going to hire that editor every time you rewrite your manuscript?
Spend the time to master the language before submitting your work. Build up your internal editor. This is part of mastering the craft. And actually, I'm suspicious that if you haven't got sufficient mastery, you aren't reading enough. You should be able to hand over a manuscript with, well, how about one single misspelling.