Don't worry about spans while you're trying on. It'll be WAY too hot. Just wear undies that cover your bum because someone (likely the consultant) will be helping you in and out of dresses. Save the spanx for when you go for fittings.
Same thing with bras. If you have a nude-colored strapless bra you could wear it (or bring it) but most dresses have enough structure that you won't need it, and it'll just make you hotter/sweatier. I wouldn't go buy a special bra just to try on dresses. Again, once you've selected a dress and know the shape/cut, you can go nuts on undergarments. It's not worth spending a lot of money until you know what you'll be working with.
I would make sure you bring some wet wipes and bottled water because you will get hot and sweaty, even in november.