I've aquired a few books over the years and advise against paying anyone to edit your work prior to submission, because the publisher will edit your work once they've signed you, and there's no point paying for something that's going to be done again for free anyway.
I understand your reasoning for wanting those first few chapters to be tight and gorgeous before you submit: but if the rest of your ms isn't as gorgeous, then agents are likely to be disappointed and reject you when they realise that. And a good editor can't edit part of a ms: they need to work on the whole thing to see the plot holes, the inconsistencies, and the weaknesses it contains, in order to provide the advice you'll need to correct those problems.
You will be far better off learning how to do the revision yourself. It's hard, but it will serve you better in the long run. Use AW's Share Your Work section; use beta readers. If you're not satisfied with that then you could use an editorial agency, which will give you a few pages of editorial notes in return for a few hundred quid: I know people who offer these reports, and people who have paid for them, and know they work and are good.
And bear in mind that with editors, you tend to get what you pay for. So if you're working with an editor whose services come cheap, you're not likely to get the best-quality work. I've seen perfectly good books turned into turgid monsters by poor editors. Don't risk it.