Do a search. This topic has been discussed to death. Your luggage won't leave the airport until you check in for ME, if you miss your flight they don't want your luggage at your resort while you're still home.
You might be able to check in for ME then leave and get your rental car but it would be easier to either divide your party in half, half take ME and the other half get's the rental car. See who gets to the resort first. Probably makes more sense to just rent a car at WDW.