Northstar, regardless of size, your choices of steel frames is limited--I know because I went through this same process last year. I ended up with a 50 cm Bianchi Eros Donna. Admittedly, I don't have much to compare it to, but I've been happy with the bike. I've upgraded a few things along the way, but it's serving my needs very well. I, too, like long rides and glad (given my budget) that I went with steel.
For some reason, Bianchi's WSD bikes don't come bigger than 50 cm, so your bike shop is correct that you're limited in that regard. If you opt for steel, I think the Vigorelli is a good choice if it otherwise fits you. It's a nicely speced bike, although I would note that it comes with a "compact double" cassette. Depending on what you want with respect to gearing, that might not be the best choice for you. You might also check to see if they can get their hands on a 2006 Bianchi Veloce. It's similar to the Vigorelli but comes with Campy Veloce components instead. If it had fit me, that's the bike that I would have purchased. I'm a Campy girl and, for mostly emotional reasons, prefer the bikes Bianchi offers with Campy.
There are, of course, other manufacturers that have a wider selection of WSD bikes--mostly aluminium and carbon. LeMond's line really changed for 2007, but you might look at their bikes. They have only one steel frame--the Sarthe--on offer now, although it does not come in a WSD model. Their WSD bikes go up to 53cm. Depending on your proportions, I suppose that could work for you. The Chambery or Alpe d'Huez look like good choices (the latter also comes in a WSD). I was with a friend when she test drove the WSD Alpe d'Huez. She ended getting the full-carbon Versailles, but she otherwise liked the bike and thought it rode better than her all-aluminum Giant.
You're right that fit is the most important thing, not a bike's designation as WSD. That said, if you buy a men's bike, you may have to modify a few things to optimize fit. For instance, the bars might be too wide or you might have trouble reaching the brake levers. I would suggest finding the right frame first and then seeing what your LBS can do to tweak it. Don't be afraid to haggle a bit.
Remember to factor in the other things you're going to need/want to get in figuring out your bike budget. It's sad to say but my bike was cheap compared to all the stuff I've purchased to go with her!
Good luck and keep us posted on your decision.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher