At $150 for the week, I would suggest getting the rental car, too. You will get to Epcot, HS and AK much faster using a rental car rather than Disney transportation. Disney buses make sense for MK, since you get dropped off at the entrance, although you will be able to take the boats as well.
I have found that Costo Travel on-line frequently has the best pricing on car rentals.
I am curious which rental company that price came up for and what location you will be renting from (MCO, CCC, Dolphin, other). When we were there last month, we picked up at the Alamo Dolphin location, which was really convenient, since we were staying at BWV. I suspect there is better pricing for off-site locations, but I have not checked that out.
Have a great trip!!