We hadn't completed our
DVC purchase when we went last December, so I can't comment on resort availability. I would assume that you need to get right on it if you want to stay. This is a busy time of year for DVC usage.
As far as the parks, we were there for Wednesday through Sunday the first week in December. During the week it was pretty slow to moderate in all the parks except Epcot. Epcot has a lot going on for the holidays in the World Showcase, so it was always busy, especially on the weekends. You could barely move around it was so crowded when the weekends hit. This meant less crowds though for the other parks though.
It is a really neat time to go to Disney. There are so many neat things to see. I would recommend as a definate thing to do at least once. Good luck.