Crowds that time in Jan are for some of those days similar to September. The first couple will be a little more (not much) crowded because of Marathon weekend and the last two days will be more crowded because it begins MLK weekend but nothing unmanageable.
March 4 to 11 will be significantly more crowded although still not overwhelming. First few days of March are not bad but as soon as you the first full week you start hitting real spring break weeks.
As to weather, that March time is usually low 70s/high 60s but can get some cooler days (50s) or hotter ones into the 80s. January can vary substantially. Have been that time when it has been high 70s/low 80s all week with no rain, when it was mostly mid-60s but one or two 40s day and some rain, and then there was January 2010 when we were there for 9 days:
Entire time highs in 40s with one low 50s. 20s at night and early morning, got up to high 30s by noon, mid to high 40s by 2 and then as soon as the sun set at about 6 temp dropped to the mid 30s and a few hours later you were close to the 20s. Coldest I have ever been in Orlando on consecutive days and it basically had about 16 days in a row just like that which we kept hearing was one of the or the longest stretch of hitting sub-freezing temps on consecutive days that Orlando ever had.
As to booking 7 months out you may be blocked out at BCV (some chance), VWL (some chance), BWV standard or boardwalk view (less chance), BLT MK or standard view (highly unlikely), or AKV concierge ( highly unlikely) or value (less chance).
