Rental cars at MCO are a breeze if you have an in terminal vendor. If you dont have to wait for a bag you can get from the plane to on the road in about 30 min. Make sure you have "skip the counter" on your car reservation too as you can avoid a potentially slow line at the car vendor. I seem to get behind the Griswolds after they loose their travelers checks or 3 families that have never rented a car before or someone who argues about not having a credit card/insurance...I digress....
I did this in October and beat the Magical Express by well over 30 min. Landed at 4:30p and was in our Poly DVC studio by 5:30pm on a Saturday. I had the family take the ME so that we could check luggage all the way through and our son still loves the ME. I had time to unpack the
Amazon order before our evcening was to begin. We had a tight FP+ planned for 7DMT followed by BTMR; however the torrential rain that day derailed our plan and closed the rides. We would have made a 5:30-6:30p FP+ (barely). The rest of the trip went off without a hitch or bad weather!
Baring unforeseen traffic delays or weather you should be fine.
Good luck.
Mike