Most of the "major" rental agencies are now on-site at the airport, including:
Alamo,
Avis,
Budget,
Enterprise, Hertz, National, Dollar, Thrifty, Firefly...I'm sure I missed a couple. For those, you won't have to take a bus at all...just check in at the counter, then walk across to the garage and get your car. Several agencies have ways you can checkin online in advance and just head straight to the garage without stopping at the rental counter in the terminal. Procedures vary, but you often will need to sign up for their frequent renter program, which is free.
You probably can save some money by renting at an agency "off-airport". For those, you will need to take a shuttle bus to the agency. Unless you're saving a lot of money, I'd recommend staying with an on-airport agency. Off-airport will slow you down getting to Disney and you have to be especially careful and build in more time upon returning the car, since you'll have to be shuttled back to the terminal.
Once you have your flights set up, I would go ahead and book something, just so you have a reservation. Do not prepay anything! Major agencies will not even take your credit card to hold a car...all you need is your name, email address and reservation times. The rates will probably be high when you first look. Don't panic, but continue to look. In most cases, you will see rates drop as the trip gets closer...often you'll see the lowest rates about a week to three weeks before you leave. During that period, and even up to departure day, you should check rates frequently if you want to save some $$$.