I suspect that because we haven't really been involved with this area of things nobody knows what to say in response.
Since this site is basically volunteers who love making maps or at least looking at maps it's not as if we relish the idea of getting involved in the transaction between a client and artist. That said, I think I understand where you are coming from and although we are a great bunch of folks here this is still the internet and one can't be too careful.
There have been conversations as to what we could offer to expand into the area of commissions but then it becomes more of a business, so as of now, what you see is what we got. (I'm not the authority on everything here so keep that in mind as you read my comments)
The best advice I could give on this is to look at the artist's reputation here. If they don't have much then they are either new to us or not involved a whole lot. Also, you might read up on some of the posts made by whoever you are considering and get a feel for what sort of character they have. Remember they don't have to be all warm and fuzzy to be productive. If you hire someone who just signed up here well, I would say that's you taking a bit of a risk because we don't know them either.
Usually, there is a partial payment upfront and a final upon receipt of the goods. You as a paying customer could portion that out however you wanted. Especially if it's going to be a long process. Remember too, the artist doesn't know you from Adam so they are putting themselves at risk by taking the job and putting in the time. Really, you have more ability here to check us out than we do you in most cases.
You are also more than welcome to have the artist create a WIP thread for the project so they can post things as it moves along and there can be an open dialog here on the site. Of course, that makes it public knowledge which doesn't work for a lot of projects. It would also mean you would have more input from the peanut gallery here which may or may not be a good idea. It is always best to have specific deliverables agreed upon before the project starts. Any experienced individuals will agree with that.
Anyway, maybe some of the others will add their 2 cents in. Either way, I hope you are able to find what you need here. I know there are a lot of satisfied customers.