I'm sure she won't have too much trouble with driving it after a little practice. They steer in a similar way to a bicycle, with a sort of tiller out in front of you. So where you think you are steering it is where you go. A power wheelchair (which my DD has) is harder since you drive with a joystick and it's easy to steer it somewhere other than where you mean to. We're trying to get up our courage to take DD's power wheelchair when we go in March. She's a good driver

, but I have visions of her escaping from us and running people over
MK is probably the hardest to get around with a wheelchair or scooter since it's busy and the paths are pretty narrow. AK also has narrow paths. Epcot would be the best place to start out in a scooter since it's more wide open and less crowded usually.
If you haven't already gotten info about where to rent one, here's a list of places DIS posters have reported renting from:
Care Medical:
http://www.caremedicalequipment.com/
Walker Mobility:
1-888-726-6837
www.walkermobility.com
RANDY'S Mobility is in Kissimmee 407-892-4777
http://randysmobility.com/
Colonial Medical
http://www.colonialmed.com/about_cms.html
Most of the sites have good pictures so you can see what they look like. I've been told the outside rental company ones are easier to drive than the ones you can rent in the parks.
