Here is a list of the places that DIS posters report renting wheelchairs and
ecvs from:
Walker Mobility:
1-888-726-6837
www.walkermobility.com
Care Medical:
http://www.caremedicalequipment.com/
Phone (407) 856-2273, Toll Free U.S and Canada (800) 741-2282
RANDY'S Mobility is in Kissimmee
(407)-892-4777
http://randysmobility.com/
Colonial Medical
http://www.colonialmed.com/about_cms.html
(800)747-0246
Some people have had good luck getting a wheelchair for free from their WDW resort, but, as Figaro mentioned, they are first come, first served and can't be reserved ahead of time. Because of that, some people have posted that what was available didn't meet their needs (either not good condition or too large or small size). People have posted that getting a suitable replacement could be quick (hours) or slow(days). So, if you do choose to get one from the resort, it's a good idea to still take these phone numbers a long.
Now, as for wheelchair or ecv, there are advantages and disadvantages to each. A wheelchair needs to be pushed by someone, which can get to be tiring. An ecv doesn't need pushing and gives more independence to the user. Most people do fine with the ecv, but if your MIL is the timid type and doesn't drive a car, she may find the ecv is too intimidating for her. If she has vision problems, she may not see well enough to drive safely.
A good way to test out if she would like to drive one is to try one out at a local store or mall. Most Targets and Walmarts have one she could use for shopping. Those are much larger and more difficult to drive than any you will rent in Orlando. So, if she does well with that, the rental ones will be a piece of cake. When you get the rental one, have her take a few minutes to just practice things like making turns (sharp and gradual) and backing up. She will be more comfortable in the parks if she has practiced before. And Epcot is a good park to go to first with a new driver. There is lots of space and the walkways are wide.