If you have a Costco membership you can go on Costco.com/travel. I always fly into Ft Lauderdale first to visit family, then drive up to wdw and return rental after resort check in.
They use Alamo, Bugdet or
Avis. I prefer Alamo because I can return it to either CCC or Dolphin. The other 2 have desks at DTD, Doubletree or Hilton.
One word of caution, if you plan on using the sun pass feature on the car, if you end up going with one of the smaller or "discount" car companies, be sure to check on their admin. Fees. They tend to be a lot higher then Alamo and budget. I like these 2 companies because they cap their fees, so after 3 or 4 days I think, or $14 you don't pay any more fees. Some companies don't have this. And there are a lot of tolls between Miami & WDW!! I like being able to use Sunpass lanes rather then stopping at every toll booth. And I prefer using the Turnpike all the way. Much nicer drive.
I forgot to add, if you don't have Costco, or CAA, check out Mousesavers.com website. They have discount codes posted.
If you have any questions feel free to PM me.