National has a rental station on site, you pass it when riding the bus out of the Magic Kingdom just before the soutbound ramp joins the northbound ramp and becomes World Drive the main north south drag. Disney buses don't go there, you have to use National's courtesy shuttle which requires appointments and has limited hours, or take a taxi there.
Because they're the only game in town, you may not have much of a choice of discounts.
However, often they let you pick up a car at the airport and drop it at Disney, or vice versa, then just the driver may be able to walk to the far reach of the MK/TTC parking lot and catch a tram.
For a party of 4 or more, renting a car at the airport for the entire stay avoids the cost of airport limo or the stops and delays of the Mears shuttle, and this offsets the rental cost a lot. Adding the convenience of the car for less expensive off site meals, many folks find it worthwhile to rent a car for the entire stay.
More Disney tips:
http://members.aol.com/ajaynejr/disney.htm
9/65
Disneyland
3/75 (World Inn* off I4 near DD)
4/85 (Kon-Tiki* on 192)
'80s Disneyland once or twice
7/94 POR
9/97 ASMu
11/98 ASMu
12/98 (Knights Inn on 192)
9/99 ASMo
12/00 ASSp
9/01 ASSp
11/01 ASSp
*no longer exists