One thing to check also is (when the motor's warmed up) turn on the heat with fan on high, and smell the air coming from the floor vents. If it smells even remotely musty or like an air freshener on steroids, FahgetaboutIt!

 

You could get one of those small inspection cameras on a cable.
Push that down inside the doors (usually can squeeze it past rubber edge) to look for waterline.
Push that camera up under the dash, look at the seat bottom, put it in a gap between fender and fender-liner.

Transmission and axles with water inside would be hard to spot..

 

Southern vehicles should not show significant rust over a couple of DECADES. That's not normal. Especially not rusty brackets or screws on the inside of the vehicle. Undercarriage rust just isn't normal on a Southern vehicle. We do not get much ice and snow and the roads never get salted. This is what causes rust. I've got a 17 year old Camaro with zero rust on the underside of it. Even the suspension components have no real rust to speak of. They are just dirty. After 9 years, nothing on my GTO. After 8 years, nothing on my 2008 F-150. One year, my drive shaft on my Super Duty shows some surface rust. That's it.

 

What's all this talk about rust? If it was submerged for months/years I would expect rust but a few hours/days/weeks of water?
If it was salt water like the ocean rising that would make sense. But outside that I wouldn't expect rust from a flood. Maybe I'm wrong...

if it runs good, and doesn't smell bad, I'd buy it even if it DID have a title indicating flood.

Maybe my ignorance is showing. It looks like a lot of speculation on this thread.