I'm a member of both the THIS and THAT writer organizations, and have the opportunity to go to monthly workshops and accompanying conferences. Just recently, we had a successful professional editor speak to us, and a question was asked about the benefits of having a manuscript professionally edited before querying agents. She responded that she believed agents would prefer to see the work pre-editing to determine what type of skills the writer actually possessed, especially if the submitted work was to be part of a series. We were a bit surprised at this statement, so I wondered what your take would be on something like this.
Would you, as an agent, rather get a professionally polished submission, or one that has been somewhat edited by the writer?
The absolute bottom line: I want to get something I can sell. How it arrived at that point is of less concern.
There's a lot to be said for getting independent professional eyeballs on your manuscript before you start querying. By the time you're ready to query you've read every page more than a dozen times and what may be crystal clear to you can be rather less so to those of us reading for the first time.
So, in that instance, I think it's wise to get that help. Whether it's a paid editor or someone else doesn't matter as much as the results.
If you have an independent editor and s/he is suggesting major structural work, or pointing out plot problems in a novel you thought was finished, you may not be ready to query. It's one thing to get second eyeballs to find plot inconsistencies or small mistakes. It's entirely another if s/he's finding things you should already be able to see.
That said, this is how you learn to write. Write, get comments, revise, get comments, rinse, lather yourself into a frenzy, repeat unto death.
The ONLY concern I have if you have professional help is that you be able to write that second book on the contractual deadline, and that deadline is about a year from when I sell the book. Bringing in an editor and doing any kind of revision takes time, and time is going to be in short supply on book two.
But, the real answer is I don't much care how your novel got into its present condition. I only care that it's terrific and I can sell it for wheelbarrows full of cash.