We always seem to do Epcot first as they are open the latest upon our day of arrival. That way I don't feel that we have wasted a day! If you go when MK has late hours though, I would pick that first!
Almost always MK, but this September our first day will be a Saturday, so I think we'll visit another park for the first day. Not sure which one, though!
On how much time I have that day. I drive up from Miami. This time... I may end up arriving at the park around 1, hungry and cranky. Maybe a fabulous lunch at World Showcase would put me in propper spirits