I don't have a 40, nor have I sailed on one, but I do own a 49, and have sailed offshore on a 43. I've also been on a 46 for some time. And further, I know a bit about the 423, and a very small amount about the 393. So, there you have my base of knowledge.
On the aesthetics, I don't think that's something on which anyone can advise you. You either are going to like the new line, or you won't. I personally like it quite a bit. That said, I also think the 423 is a very good looking boat too.
In terms of the interior woodwork on the new line, it does scratch more easily that I would like. We plan to build up a few coats of varnish so that scratches are in a finish that can be repaired relatively easily. The floor, however, is another story entirely on the new line. It's a composite. I happen to think it looks great, which really surprised me frankly, and its very durable. Cruising with two young boys, I can tell you that we've tested that already.
On the sailing performance, I believe the new line performs significantly better. The 423 is not a bad performing boat, but she pounds fairly mercilessly. But if you're not racing, and if you are looking at either of these boats I assume you are not, then I think the difference in performance is not likely to be significant.
I'm not sure on what your broker bases his opinion that the 40 is not popular. My understanding is that they have sold very well, and generally are considered to be the best value in the new line. Also keep in mind that the 40 has not been out that long.
The alternator is not a function of the boat. You should come to terms with the fact that stock alternators are not going to keep your batteries charged if you are going to do any kind of cruising. Daysailing and weekending it would be fine. But even for going on week-long cruises, you'll be running your engine all the time and you'll never get your batts charged. But again, that has nothing to do with the boat. You should consider upgrading to a high-output alternator and regulator, regardless of what boat you get.
On the 393, the only reason to get that boat over one of the other two would be cost. I think in every other respect the 423 or the new 40 will be superior to the 393. There's a lot of opinion in this last statement by the way, so others certainly might have a different view.
Hope this helps, and if you have any follow up or more specific questions, fire away. Keep us updated on your progress.