I agree and think late September is a great time to travel to WDW. You can never plan on the weather because it too unpredictable. I've been in September before and the crowd level is low and it's amazing. Combine that with maybe a few rain showers and you can walk right on some of the attractions.
The rain usually passes very quickly and Mickey Ponchos can be bought all over the parks.
It's always a good idea to book early to get your first pick at resorts.
I think it's a great idea to stay on Disney property too. You can't beat the free Disney Bus System!
We actually did off/on property during our last vacation and left the off property hotel a day earlier to stay an extra day at The Beach Club Resort. That's our family favorite so far!