I lived in WP Beach in the late '70s for 2 years.
One thing I can tell you, the gulf stream on the east coast, and the thermal breeze on the west coast keep the area near the shore line a very nice 70 to 80° all year round. At the same time Orlando can be 100°.
Sept is the start of the cooler days, but I would still recommend you hug one coast or the other, and I would go with the east (ocean) side: Daytona, NASA, Jupiter, WP Beach, Miami...
Even just 3 to 5 miles inland the temp can be 20 degrees higher. That is why "beach front" property is so desirable.