Well, I will concur with what others have said. If you are going to be on the trail for four to six months, miserable sleep will make the rest of your trip awful. I would suggest that on occasion crawling into your sleeping bag will be the day's highlight! That isn't a knock on you, the AT or hiking in general, just when it's cold, you're exhausted, your stove stops working and you are forced to eat cold ramen for dinner, a warm bag is a welcome respite.
That much said, here are some suggestions....
ON sale now at Altrec, a 20 degree, 800-fill down bag by GO Lite (in regular length) for $211.00.
Mountain Hardware has its 32-degree rated Phantom for $190
Yeah, considerably more than you would like to spend, but in my mind, excellent value.
Bear in mind, that it is unlikely the same bag that would serve you well in Florida is going to be appropriate for the highest points along the AT. I would personally favor buying equipment that is appropriate to the AT and buying other gear later appropriate to Florida. Who knows, maybe you become inspired by your AT journey and decide to hike the PCT, CDT, the Colorado Trail, Wonderland Trail, etc. All would require a warm, high quality bag!
Have fun and best of luck! Let us know of your decision and keep us posted on how it worked out for you!