Hi and welcome!
Crowds will be very manageable starting on Wednesday the 3rd. The Marathon is the following weekend but that really has no effect on crowds in the park (although it does have an effect on transportation and certain areas of the parks on Sunday Jan 8).
Weather is very unpredictable. You can find temperatures near freezing in the morning and highs only in the 40s-50s, or it could be beautiful with highs in the 70s. There's no way of telling what WDW weather will be in January until you get to about a week ahead of time. You will need to bring some long sleeves, some short sleeves, and some sweaters! There will probably be some clothes that never make it out of your suitcase.
Have fun!
Bob