1. Am I too old to think about eventually competing? I am a stay-at-home mom to 2 young kids and I live in Maine, if that makes any difference. I know that most of the pros out there started when they were like 5, so is it ridiculous to think that I may eventually make it there?
You're never too old to think about competing. Competing as a pro? Possibly, but I would never say never. You may be that one person in a thousand that will be a natural at ballroom and go as far as you want in a just a few years time. Oh, and some of the top pros are in their late thirties and forties.
As one other poster said, there are divisions at almost all competitions for all dancers of every age and experience. If you've only been dancing for a month, you're in luck because there are categories just for you. Been dancing a decade, same thing is true. And don't worry, they won't stick a newbie into the open pro division just to see them sweat.
2. I have to buy shoes because all I own are sneakers and boots. Any advice on what to buy?
Go to a couple of lessons before you start looking seriously at shoes. Talk to the other ladies and see what they say and also where they have bought their shoes. There is only one piece of advice I would give you on shoes. Buy two pairs. Buy a pair of heels, if you think you'll be wearing them to dance parties or other classy events. Wear them to dance class once in awhile, just to get used to dancing in heels, if wearing heels isn't something you do very often. And for your second pair, I would suggest a pair of dance sneakers. After weaing your heels for a bit of time, your feet will scream out their love for you when you slip the sneakers on. You just need to make sure to get plenty of practice in heels when prepping for a showcase or competition.
We have one teacher at the studio who wears sneakers exclusively, even to the social dance parties. The only time I've seen her wearing heels is during the showcases. Of course, maybe she wears the sneakers to protect her toes from clods like me.

She, too, is from the world of ballet and is used to wearing little or no heel.
3. The guy said on the phone that I could dress casually for class. I'm used to wearing stretch dance pants and tank tops to dance classes - is that appropriate for casual ballroom attire?
If it's like the group classes I've seen and attended, you will see everything from bluejeans to dresses, from dress shirt and tie to t-shirts. One thing you won't see is tights. To be on the safe side, wear a pair of bluejeans or slacks and a button down blouse. You will fit right in and after you've been to one class you will see what other ladies are wearing and what is appropriate. If you showed up in a pair of tights and a tank top, I can assure you, you would feel out of place. It won't be like the ballet classes you took all those years ago and it definitely won't be dance class at Julliards.

One last piece of advice, have fun! And try to formulate some scheme to get your husband to go with you. Dancing is almost always better when you have someone to dance with.

Dave
http://www.BallroomDangers.com