Well, a rental car would be handy but how many nights are you going for?
Ask the hotel questions like how often do your shuttles run, since it’s summer & pressumably busy do they add more shuttles into the loop if they get full? I’d hate to see you miss a shuttle & have to wait another hour or something b/c it filled up with guests waiting before you.
Otherwise, I agree with cjtownman. If you are going to stay off site AND pay for a rental car then just stay at a Disney economy hotel & skip the rental car 'cause the buses are constant.