That time of year gets a bit crowded, but I find it to be tolerable. You can minimize line by judicious use of fast pass.
Weather in Orlando from mid June to September is almost always the same, sunny, high 92 to 95, humid, and a good chance of "5 o'clock Charlie", the 15 minute thunderstorm. Unfortunately, the short thunderstorm actually increases the humidity as the puddles evaporate. It's the humidity that can make it unbearable. Find shade and AC when possible, drink LOTS of water, and don't overdo it if you are not used to the heat/humidity.