Hi Kirstin,
Lets get the easy things out of the way first! The Princess Marathon will add a few more people to the parks however the races take place early in the morning so traveling to and from the parks is slightly impacted. If you are traveling to the airport via Magical Express, you might have to leave a little earlier to maneuver around the closed roads.
As for the temperature, it can really vary in the winter. It can get as cold as the 20's but as be warm as the 70's. I always check the online weather sites for the weather right before I leave so I know what to back.
You should be seeing deals now for your time period. I personally think the room deal is better than meals especially if you are not be eaters or do not like spend time sitting down to eat. I also think there are easy was to save on meals such as eating breakfast in the room with cereal or foods from home, snacks and water for the park. Also, some meals are large that you can share them.
I would sign up for Disney's Facebook and Twitter pages as they often release deals to fans prior to the general public. Also, Mousesavers.com is a great site to look at packages. Finally, call Disney and ask them for help. Cast members are very knowledgeable about packages.
Wishing you a magical vacation!
Beth