The OP has said they've decided to go to Chicago instead, but I wanted to add my 2 cents about the the heat and crowds.
We only have availability to go a peak times in the heat. I was initially nervous about that, but I have to say I have been very pleasantly surprised. I'll use our last trip from a couple of weeks ago as an example.
We were in the parks when crowds were at an 8. We used Josh's touring plans/models/examples to generally follow for our days there. We were at rope drop for 5 of our 7 park days, took long, leisurely pool/resort breaks in the afternoon, and went back to the parks in the evening. We were able to ride/see everything we wanted, never had to wait more than 15 minutes for anything, were not in the parks during the heat of the day with the exception of our arrival day when we got to MK at 2:50 and watched the Festival of Fantasy parade from the train station in the shade and then took the train around to Fantasyland. The evenings were the best, honestly. During/after the night shows, ride waits decrease DRAMATICALLY. We watched Celebrate the Magic/Wishes/MSEP on the first day we were there, and rode Splash Mountain twice, BTMRR, Haunted Mansion, Space Mountain Twice, and 7DMT twice, and much much more. The evening EMH at Epcot were great! So relaxing walking out of World Showcase in the evening into Future World and riding there. We did EE, Safari, and saw FOTLK at Animal Kingdom before a late morning breakfast at Tusker House. Walked on Tower of Terror (2 Xs) and RnRC during the first Fantasmic, then was able to sit in the 11th row in the center for the 2nd Fantasmic.
The only time the crowds were really noticeable to us was during Wishes. We watched from MainStreet for the correct view because there were no fast passes available. The street was crowded. But there was room to watch...I wouldn't let this deter me from attending,
We did much, much more, but I wanted to give hope to those that go in the summer. We had a great experience. Don't let the weather and crowds discourage you!