Personally, I don't recommend hiring editors. It doesn't help in the long run. OTOH, if you're looking at writing only one book for a specific purpose, never to write another, then it might be a good solution.
But here's the thing with long-term writing goals in mind.
If you hire someone to do your editing for you (all "you"s in this post are the generic, non-specific "you"
), it's not likely that you'll learn how to make your writing any better. You'll churn out the same quality, book after book, that still requires editing.
But. If you learn how to edit your own, then guess what happens? Your next book's first draft becomes better than the first, the fifth is better than the fourth, the tenth is better... rinse and repeat. If you're in this for the long haul, you're much better off learning how to edit yourself. That, and it saves a whole lotta money.
So how do you learn?
I would suggest that, if punctuation, spelling, and grammar are weak areas, take some refresher courses. Whether that's through ConEd or the local college or online or by reading Strunk & Whyte's whattheheckisitcalled, it probably doesn't matter much.
I would also suggest that you critique other people's writing. It doesn't matter if you've never done it before - everyone has to start somewhere and guess what? When you crit someone else's novel, you learn A LOT about writing, what works, what doesn't, and how to improve your own. In fact, it's been my experience that people tend to learn a lot more by critting other people's stuff than they do by reading the crits that other people give them.
Crits can be varied, anything from an overall this plot doesn't work for me because to a line crit analyzing every single cotton pickin' grammatical error or weak verb choice. The more you do it, the better you get at it.
As you do crits, you'll find that it becomes a lot easier for you to notice the problems in your own work, therefore editing becomes much easier, and guess what? Your first draft improves dramatically, too.
Good luck!