OMG you are me, just one year ago!
My advise, watch the local weather forcast the week before you leave! We were there(with the hords) last year for c-mas through NYE. The weather was 40 degree's(and don't fool yourself that 40 is different in FL..it is a REAL 40!) one day and 80 degree's the rest of the time.
You know what worked best for us...silk long underwear!!!!!. Easy to pack and very warm!!! We brought layed jackets(a sweatshirt, a sweater and a windbreaker) and wore those going to the airport from home.
The weather is all over the board down there in Dec and January. One day we were in ALL the clothes listed above and 2 days later we were going into the Hall of Presidents(not a usual attraction for me and my 2 kids 11 and 13) just to cool off and get into the air condtioning!!!
Good Luck and Enjoy your Vacation ~ Debbie