Have you considered Tampa (TPA) ? MUCH better airport experience IMHO, and only 60 mins up the I4 to Kissimmee area. We fly to TPA, but in fairness we hit the beaches for a few days before going up to Kissimmee. You may get different prices from there.
If picking up an Avis car, be sure to join Avis Preferred to avoid an awful wait in the main terminal (MCO or TPA) to get your car, and the upselling nonsense, go straight to the garage and take your car.