We went the last week of Sept. last year and it was definitely hot (high 80's and even low 90's). We were lucky and happened to go the week in between hurricanes, so there was no rain--it rained once for 20 minutes only, and that was our last day when we were already back at the resort. You should expect some light daily rain, though.
The absolute best thing about this week was that there were NO LINES for anything. We literally walked on to every single ride. We did the Buzz ride in MK 7 times in a row, just kept getting back on. The only thing we actually waited for was Dumbo and that was a whopping 6 minutes. Have fun!