I second the "pack a little of everything." We normally go in December. I remember one Decemeber with my mom FL had record colds (I think much of the orange crop was damaged with frost). We bought coats, gloves, hats and blankets (for the parades, Fantasmic, etc) - it was not cheap to buy this all at Disney prices. The highs some days was not even 40. Even this year, we had some days in the mid 80's and some days in the mid 60's for highs. I know that's Dec and not Jan, but I think Jan is just as variable as December.
The other thing I've found is that even if it's much warmer in Orlando than at home, if I don't have the right gear, spending a great deal of time walking outside will not be fun.