Most down allergies are really allergies to feathers, not an issue with high quality (800+ weight) down. If you can borrow or rent a bag with high-quality down, try one out to be sure first. I've had problems (A-CHOO!) with feather pillows, but not with my Western Mountaineering sleeping bag. A high quality down bag is an investment for a lifetime (if well cared for), while even a high-quality synthetic bag usually lasts only 5-8 years. Someday someone will invent a synthetic insulation that can be compressed and recompressed many times without damage, but it hasn't happened yet.

It's generally advised to get the pack last, so you know your gear will fit inside and that the pack carries comfortably for you with your gear (and the equivalent of a week's food and a day's water) inside. Pack fit is almost as individual shoe fit, so be sure it works for you. See the articles on the home page of this site, if you haven't already, for lots of tips on gear selection.