I agree with bigun's comments. It is VERY hot in Florida at this time of year, and tropical storms are fairly common. Even without a storm it will probably rain very hard every afternoon for a while, which raises the humidity to 100%. It can feel very steamy afterwards!
You will not be outside all the time - many rides and attractions are indoor with air con, and have fans spraying a cold water mist in ride queue lines at intervals.
They sometimes have to clear the water for a while in the waterparks if there is a risk of lightning, until the danger is past.
We have not been in July/ August, but have had temps in the high 90's in April/ May and also some stormy days, but they don't usually last long. We had to divert to Tampa airport on arrival one April trip, as there was a tropical storm and it was too windy to land in Orlando, so this can happen anytime.
We also use the water bottles with the fans attached, which have a shoulder strap to carry them with. We bought ours in the parks.
The parks are always busy when the Americans are on holiday - we hit Spring Break by accident one year, which was horrendous! Water parks were full within 30 mins of opening, so you had to get there very early to make sure you got in. Queues for slides etc were very long.
As bigun says, you need to be at the park gates well before opening time, and head for the most popular ride to get Fastpasses, then go straight to another popular ride befor that gets busy. You can get another fastpass after you have been on the ride or after about 2 hrs if they have not all run out for the day.
When we last went, you could buy Fastpass/ Fast Track tickets at the entrance of Universal Studios and Islands of Adventures parks. We didn't need to do this as parks were not too busy outside school/ college holidays, but I would buy them if parks were crowded to avoid long queues.
People make dining reservations for table service and buffet restos in the Disney parks and hotels from 6 months in advance so some of these could be booked up by now. We usually just eat in the fast food ones as we go, although we do like Tuskers buffet for lunch in Animal Kingdom park.
The Ponderosa restaurants are all you can eat buffets in the Orlando area - there is more than one. They have gone downhill a bit since we last ate in one, ( so I have heard ) but still good value for money.
We have not hired a car as we stay in a Disney hotel or with a friend nearby, and use the Disney buses and taxis. If you are driving, arrive early at all the parks to avoid long queues for the car parks.
Taxis can drive straight in, and drop you by the park entrance. People driving have to park, then usually catch a land train to the park entrance as the car parks are huge.
Not meaning to put you off regarding queues and crowds, I am sure you will have a great time. Just be prepared for very busy parks, and the very hot and humid weather. Worth packing some Magicool sprays to take into parks with you, as these are so refreshing - you can buy them in chemists. We take small telescopic brollies which double up as sunshades.
People on the Trip Advisor Walt Disney World and Orlando forums can help with tips and up to date info.