No not all the buses will have lifts. 95% of Disney's buses are lift equipped though so it isn't often when you get one that isn't equipped.
Only once did I get really stuck and actually rudely treated on my past trip. First bus not equipped, 2nd bus stpped then drove off without me, 3rd bus full and boy was the driver nasty when I commented that he was the 3rd bus to pass, finally a 4th bus picked me up and was very apologetic that I had to wait so long.
Buses run every 20 minutes. There are 4 or 5 different buses that will come in the 20 minutes before the next one for the park you want.
I would suggest you request building 14, 15 or 16 which are close to the main building and the first stop for DxL. That way you have less a chance of the 2 wheelchair spots being filled before it gets to you. It only stops at PO before getting to that spot. It also avoids having to cross any of the bridges to get to the main building and those bridges and rough and uncomfortable to ride over.
Most of the time you won't wait more than about 20 minutes. Figure another 5 - 15 to get to your park of choice once you are on the bus. The longest it has taken me to get anywhere in WDW has been 1 hour and that was from Ft W to Boardwalk via TTC early in the am and transportation had to call the bus for Boardwalk to come pick me up.
Pooh