Chris,
Congratulations on your call! Here's my answer to your questions:
1. They have ATMs in many locations in the larger cities (most likely in whatever city you'll be serving in.) You can use the ATMs just like you do here in the states. But I believe that most banks will charge you a "transaction fee" or "currency exchange" fee, so you'd want to find out before hand what the details are. When I was a missionary, the church issued each missionary a debit card, which was refilled with our monthly allowance, so you probably don't really even need to worry about bringing your own card.
2. I don't recall A/C being very popular when I was there ('99-'00). I'm not sure if it's changed since then. But don't worry about it. Many a missionary has survived without A/C, and you can too!

Besides, don't you want some bragging rights when you return and everyone's telling stories about their mission?

3. Yes, compared to the U.S., food and clothes are pretty cheap in Bulgaria. You can eat a decent meal at most restaurants for under $5. Mmmm, I wish I could get some of that food now...
4. When I was there, there were about 120 missionaries in the mission. They serve in many cities across the country. Read the
mission history page on the web site for more information.
Let us know if you have any other questions!