Items to consider for January-
Our Florida weather is very dependent on the fronts. Most days it tends to be very comfortable with lows in the mid 50’s to highs near 80. If a front is from the south, tropical, expect very nice up to warm and humid weather. Fronts coming from the north can drop the highs to below 50. If it is cool, because there can still be humidity, it can feel colder. Expect a few days of each through the course of a week.
The week around the holiday are usually crazy busy. Longer park hours can help if you are willing or able to stay late or arrive early. Expect crowds and some waits but you can get to more things.
Marathon weekend crowds are a little different. There is an increase in crowds but key is travel logistics can be impacted between locations on property or in parks. Epcot and Magic Kingdom probably are most crowded around that weekend, especially the Monday after.
Mid to late January are usually not too bad for crowds but park hours are a little shorter. Also check for and see if rides are scheduled for maintenance. It is not uncommon for the parks to do updates or routine Maintenance around this time.