There is nothing crazy at all in WDW in September.
September sees some of the lowest crowds of the year in Orlando. WDW is deserted - hence the
free dining plan offered the last few years, lowest resort rates, shortened park hours, and many attraction rehabs scheduled for Sept-Oct.
The busses will be somewhat LESS crowded in September than the rest of the year. Of course, since there is less demand, Disney will put fewer busses on the road, saving them money and manpower, and giving them opportunity to do routine overhauls and upgrades to the bus fleet. But even so, you should have no undue difficulties with bus transportation at WDW in September.
That being said, I prefer to rent a car whenever I go to WDW, whether I am on-site or off, no matter the time of year. I prefer having the freedom of a rental car, the flexibilty, the 100% guarantee that I will always get a seat and a seatbelt, and the shorter travel times from one place to another.
If renting a car will not strain your
budget too much, I always recommend it.