I would not buy a flood car, it will have many electrical issues.
I have "rescued" several flood cars. After Harvey several friends and family had cars that they didn't have the desire or ability to mess with, that I either fixed for them or bought and revived. They can be exorcized of gremlins but it can be a process that is not for the faint of heart.
I started by removing the interiors and letting them dry in the sun for several days, also the doors open on the car sitting in the sun for several days. Unplug every electrical connector that was beneath the water line, spray it out with contact cleaner (you'll need to buy the stuff by the box), scrub with a toothbrush if any corrosion has already started, and pack full of dielectric grease. Same for every single module (airbag module, fuel pump controller, etc, all the modules that got submerged); open the module, inspect for corrosion, clean with contact cleaner, coat circuit boards with dielectric grease. It is a good idea to hit
every connector and module, even those that weren't submerged, because ambient moisture (it's like a rainforest inside those cars) can get into the connectors. But this is an even more daunting task that I chose to forego, and luckily it did not come back to bite me.
Put the car back together, drive it for a while and address the gremlins that pop up. After a couple thousand miles usually what's going to come up will have come up already. And usually it can be remedied by repeating the cleaning and greasing of the effected connector/module. I only had a couple of modules that were actually damaged and had to be replaced.
The cause of the gremlins is almost always bad connections (intermittent open circuit) caused by corrosion in connectors or intermittent short circuits between circuit board traces, also caused by corrosion. The corrosion is caused by electrical charge in the presence of moisture. The best thing you can do if your car gets flooded is disconnect the battery ASAP. You take away the electricity and the corrosion will be minimal. Do not reconnect the battery until the car is
completely dried. If the windows get foggy when sitting in the sun, it is not dry enough to reconnect the battery.
All of the cars I repaired are still on the road and not having issues. I consider that a stroke of luck, and only possible because my first action after flooding was to do what I did. Where it becomes an insurmountable problem is when a car gets flooded and the owners thinks "hey it still runs, guess i dodged a bullet," and they begin to drive it around without addressing the moisture in connectors and circuits. They're driving around pumping more and more electrons into the many thousands of little reactions which are slowly eating the car like a cancer. By the time it's discovered, it's progressed to the point that it's a lost cause.
It really is a crapshoot buying a running/driving flood car. You don't know what steps were done to mitigate the damage. I would only buy a flood car that I personally rescued from a flood or that I know hasn't been driven or ran since being flooded. And even then, I would not pay even half the blue book.
