Are agents interested in first-time authors with a good product or generally speaking do they prefer published authors only? Are first novels worth the same to publishers (given the apparent risk of the author being a one-hit-wonder or not at all) or are agents able to negotiate better deals than the novices? I don’t know whether to send my completed MS to the editor who requested to see it 3 years ago (she read an incomplete version & followed me up twice by email soon after the birth of my 2nd baby with an open invitation to send it again when completed—but I’m embarrassed at how much time has elapsed—the novel only took an aggregate of 2 weeks to finish but with 2 young babies, 2 businesses & 2 house moves I wasn’t able to commit to the writing life at the time).
There are a few different questions in this post, so let's break 'em down:
1. Are agents interested in first-time authors with a good product or generally speaking do they prefer published authors only? All published novelists were first-timers once - if publishers and agents were only interested in published authors, we'd be out of business in about 15 years' time. I can't speak for all agents, but I'm always interested good product regardless of how published or not the author is. For reasons discussed in other posts, publishers and agents are slower to take on first novelists than someone who has an existing readership, but 'slower' doesn't mean 'never'.
2. Are first novels worth the same to publishers (given the apparent risk of the author being a one-hit-wonder or not at all) or are agents able to negotiate better deals than the novices? Deals for first fiction are not generally worth a huge amount of money regardless of who is involved (and which country you're in); where agents tend to make a difference is in finding you a publisher in the first place, because a lot of publishers won't look at first fiction unless it's from an agent, and in ensuring that the contract is fair and you're not unwittingly signing away rights that you might want to keep. If there's more than one publisher interested, though, they can certainly make a difference in the amounts of money involved.
3. I don’t know whether to send my completed MS to the editor who requested to see it 3 years ago ... This editor is clearly interested in your writing (no one exhibits that sort of interest for the fun of it), and as she's working in publishing she's probably realistic about how long writing takes. Three years is really not much - some (published) people will take much longer than that to finish writing a novel - so she won't hold that against you. She's gone out of her way to let you know that she wants to keep talking, so why not drop her a line and say, honestly, that you've been distracted by children and moving house, but the manuscript is finally ready so would she like to see it? She'll say 'no' if she doesn't want to, but the reason most likely won't be the amount of time you've taken. Very few writers can write full-time; most are fitting it in around life.