Every time I've been has been in August and I struggle a lot with the heat, but the only time it was truly unbearable was the time we went nearer the beginning of August. The last couple of weeks of the summer holidays is totally bearable, so long as you don't try and rush around everything.
As a PP said, it will be hot and humid but there are usually lovely cooling rain showers in the afternoon (although admittedly there have been a few out and out thunderstorms too, which meant closures in the water parks). Also Disney is very well set up for the weather- you can go from air-conditioned building to cool mist stations to air-conditioned building etc to get around the parks.
We've done all sorts of accommodation options too- villa with own transport, villa with shuttle busses, onsite accommodation with dining plan. Out of them all I preferred the onsite option- very relaxed, busses everywhere, lots of food options (although I sorely missed plain cooked veg by the end! 
), but expensive in comparison.
I didn't much care for having our own car tbh- walking to and from the car parks, paying the daily charge etc. And it was a sweatbox at the end of every day- ok if someone will get in and turn on the aircon before you get in though! But you do have the freedom to go out and about then.
This year we're staying in a four bed villa offsite and planning to get Ubers/taxi/Lyft in and out of the parks (bit of a gamble, but are resolved to hiring a car if necessary).
If you want the villa option, I'd recommend HomeAway. Look for flights and villa in December this year- that's when the prices are usually lowest ( although prices were silly high this year, and I got lucky by booking at 1 am in November). Track the prices of the flights from September onwards.