The busses are 55 passenger motor coaches. The Magical Express busses are brand new models, although you may get a ride on a Disney Cruise Line bus or a black and yellow Mears Transportation bus. (Mears runs the service and uses all their resources to accomplish timely service.) The busses run videos and DVD's during the trip over to the resorts and back to the airport.
As I understand the time span for DME, the service will at least last during the 50th Celebration which will last 18 months.