I'm out shopping today, but just wanted to make a quick reply.
Follow the link in my signature to the disABILITIES FAQs thread ( also located near the top of this board)
Post 2 of that thread has information about renting wheelchairs and
ECVs.
Post one of that thread is an index to what post has what information.
Posts 18-21 have information about transfers and she will want her crutches to help with transfers.
The attractions are wheelchair accessible to the point of boarding (with a few exceptions - like Peoplemover and Swiss Family Robinson Treehouse.)
Some are wheelchair accessible, but not ECV accessible. Those will have a wheelchair to switch to.
An ECV would give her more independence, but there are attractions where she could stay in the wheelchair for to ride in an accessible ride car, but would have to transfer if she was using an ECV. So, you may want to consider whether an ECV or wheelchair will be the best choice.
If you rent an ECV, make sure the rental agent knows you want one that you can transport in s car trunk and they will rent you the correct type.