Follow the link in my signature to the disABILITIES Board and try posting your question there. You will find lots of advice from people who have rented and manouvered scooters (also called ecvs).
If she has never driven one before, she might want to "try it out" at a local store. many grocery stores and large discount stores have ecvs for shoppers to use. Those are usually slower, bigger and harder to manouver than the ones you will rent at WDW.
Almost all the buses are wheelchair/ecv accessible by either ramp or lift. It takes a little practice (have her practice backing up before her first bus ride) but it learned quickly. For the boats, she will drive straight on. Be aware though, that in certain conditions (like if they have had a lot of rain or are in a drought condidtion) some of the boats will not be accessible to an ecv because the dock and the boat levels don't match up. This is not usually a problem at Epcot, MGM or the resorts in that area, but it can be a problem at the MK and the MK resorts. The small boats (like going from OKW to DD) can't transport ecvs, but the larger boats like the Friendship boats at Epcot can.