If she has a wheelchair, I'd suggest you bring it with you and gate check it on the airplane. It's long way from the gate at Orlando to baggage claim.
If you gate check a wheelchair, you can keep it with you until you board the plane, all the way to the gate. When she boards, you fold up the wheelchair and the baggage handlers will put it in the plane and then return it to the gate when you get to your destination. Ask about gate checking when you check in to your flight - they need to put the correct tag on it to make sure it gets delivered to the gate.
It is possible to get a wheelchair lent to you by the resort, but there may or may not be one when you arrive and sometimes what they have is not appropriate for your needs (like they may have an extra wide wheelchair when you need extra narrow). There should not be a problem with being able to get a wheelchair at the parks, but keep in mind that if you do that, she won't have one at the resort and many of the resorts are huge with a lot of distance between things. The WDW resort transportation is wheelchair accessible. You can drive a wheelchair right on to the boats and monorails. The buses are equipped with ramps or lifts and tie downs to secure wheelchairs into the bus.
Most of the rides/attractions do not have special wheelchair entrances. Ten years ago some attractions did, but all of AK and the Studio and most of Epcot and MK have Mainstream Access which means the lines are wheelchair accessible and you will wait in the same lines with everyone else (not get to the front of the lines). In some cases (Kilimanjari Safari is one), you will actually wait longer when using a wheelchair.
