I love fixies for commuting, but you would need to be a pretty strong rider to push a single speed up some of those hills in Atlanta. I also never advise anyone to buy a craigslist bike unless they already know a lot about bikes and can work on their own bike. I have seen people get a "great deal" on craigslist then spend more paying the LBS to fix it up than if they would have just bought a new bike in the first place. Finally, I wouldn't get a folding bike, unless one of your requirements is to have a folding bike. I had one on a sailboat that we used for exploring local towns while cruising. It was perfect for that purpose, but I wouldn't commute on one (especially in that price range) unless I needed it to fold to take it with me on the train for example.

You're going to have a hard time doing better than that breezer at your price range. It's a much nicer bike than that schwinn for an extra hundred bucks or so, and 8 speeds should be fine for your purposes.