There is a link to the disABILITIES FAQs thread in my signature or you can find it near the top of the disABILITIES Board.
The first post in that thread is an index that tells which information is in which post. Besides dome of the information on page one, I would suggest post 18-21 on page 2 which talk about the types of transfers (how far down, etc).
All attractions are wheelchair accessible to the point of boarding with these exceptions where guests must be ambulatory:
Tomorrowland Transit Authority
Swiss Family Robinson Treehouse
Tom Sawyer's Island - anything over about 200 feet from the dock
Peter Pan - this does say guests need to be ambulatory, but that is because there is a short window of time to board and the moving walkway can't be stopped or flowed. You may be able to ride first ride of the morning or last of the night because it will be stopped then.
Power wheelchairs have the same 'footprint' and turning capability of a manual wheelchair, so pretty much everything accessible to a manual wheelchair is also accessible to a power one.