Sounds like a LOT of trouble to rent a car...by the time you ride DME, get the car and drive back to WDW I am guessing you will have wasted close to 3 hours of your vacation day. Also remember you can only ride the DME back to the airport on the day you check out of your resort...so if you are not checking out the day you want to pick up the car they will not book DME for you.
Try looking at Dollar which is in the Grosvenor right across the street from the DTD marketplace bus stop. So you can ride the bus from your hotel to DTD and walk across the street(try using the oi1054 code for a decent rate) You can pick up there and drop off at the airport with no additional charge and you do not have to pay all of the MCO related charges on the rental. There is also a Hertz at Shades of Green.
You might also want to think about bidding for a car for your entire stay on Priceline... I was able to get a mid-size car for our entire 13 day trip via Priceline for about $40 more than I was paying for the cheapest 1 week rental I could find via conventional booking (which was from Dollar at the Grosvenor BTW)