M_M, pbhunt is correct. You get your best discount by joining the Disney Club (DC), which gives you complimentary membership in the Emerald Club and therefore let's you use the Emerald Aisle service, and using the DC contract code to make your reservation on National's web site. Your DC welcoming packet will also include National (& other) coupons for you to use. I realized my $47 savings by combining the DC contract code (20% discount) and a coupon code ($20 discount) when making my reservation.
You must reserve at least an Intermediate size car to qualify for Emerald Aisle service. My experience, however, is that there is very little cost difference between an Intermediate and a Full-Size. Remember, you are only guaranteed to get a car in the class that you have reserved. If you go at a busy time and have reserved an Intermediate but really need a Full-Size and the Emerald Aisle is wiped out, National is only obligated to provide with you an Intermediate.
As far as the selection, my only experience at MCO was last September. I arrived at about noon on a Thursday and found 12-15 cars on the Emerald Aisle. The mix was roughly 3 Blazers, 1 PT Cruiser, 2 Dodge Intrepids, several 4 door Buicks (Regals & LeSabres). Of course, this is just luck of the draw and I've read others on these boards that found convertibles and minivans available on the EA. As always, your mileage may vary.
Epscot