The weather is normally in the 70's to low 80's from our experience, with a chance of colder fronts for a day or two. One year it was a record low and then it got warmer after that. You will definitely want to bring a sweatshirt or light jacket for evenings as it does get cool.
The crowds will be very busy, but manageable as long as you use FP and get there early. Mid afternoons will be extremely crowded so just be ready for it and be patient. There have been a lot of threads with regard to FP+ and it's impact on FP availability and wait times. Not sure how that will effect your vacation as it was not "in play" in the past.
Have a fun time!