The only thing you can plan on is dress in layers. The US southeast is impossible to predict. One year it can be 30 degree's with the orange groves is danger of freezing and then the next year a heat wave, the year after that perfect weather in the mid 70's.
It totally depends on that particular years weather pattern.
If it were me, I would not purchase much. Maybe a couple pairs of capris if you don't usually wear them? You'll have to wait until 7-10 days out to really know what extreme if any you'll be dealing with.
I totally understand wanting to shop and plan (favorite part of my vacation

) I would plan short sleeve tops with sweaters or jackets for the morning and evening, a couple long sleeve tops just in case, a couple pairs of shorts, couple pairs of capris, and couple pairs of pants. And of course my swim suit lol. But at the end of the day you really will have to wait until the week before to know what to really pack.
Orlando is not as far south like the Keys or Miami. It's much more at the mercy of the jet stream. It can easily be 80 degrees on your first day and a high of 45 degrees on your last.
ETA: weather history
On December 1st, 2010, the closest available weather station to Orlando, FL (ORLANDO EXECUTIVE AP, FL), reported the following conditions:
High Temp: 73.4F
Low Temp: 57.2F
On December 2nd, 2010, the closest available weather station to Orlando, FL (ORLANDO EXECUTIVE AP, FL), reported the following conditions:
High Temp: 60.8F
Low Temp: 42.8F
On December 2nd, 2009, the closest available weather station to Orlando, FL (ORLANDO EXECUTIVE AP, FL), reported the following conditions:
High Temp: 82.4F
Low Temp: 64.4F