Welcome to disABILITIES.
We have been to WDW in March, April and May. You can get hot days or weeks in any month, but since March is closer to "winter", you would be likley to have generally cooler conditions. We've been there in May when it was really hot , very humid and generally felt like a sauna, especially late in the month.
As for busy-ness, one good way to figure out how busy it will be is to look at the room prices (what "Disney season" it is). When they anticipate high demand and high occupancy, they can charge more for the rooms. Also, look at the calendar to see where Easter falls that year. Spring break time begins some time in March and goes thru some of April (schools in different parts of the country have off at various times for Spring break). We've actually been at WDW at Easter some years and it get noticably busier the closer you get to Easter. The week after is busy too.