Like the others said, a basement, even a cool one is a great herp room.
It is much easier to add heat to a herp room that is too cold than it is to cool a room that is too warm.
I wouldn't use the cages as the wall. You would only be asking for trouble.
First off, you won't be able to move them should you decide to make changes to the layout of the room. You would essentially be stuck with whatever layout you first select.
Secondly, if your house does suffer from flood damage, most likely you will have water coming in your basement and flowing down the walls. If your cages are mounted on the walls all of that water will end up destroying your cages, possibly killing your animals and shorting out any of the electrical elements you have in them.
Which brings me to the third reason: it would make changing your heating elements an additional chore.
The fact that your house is on a flood plain shouldn't discourage you too much. Sure floods happen, but they don't always result in six feet of water in your basement. (In which case, I doubt you and your family would be worrying about your cages and racks too much... your family and your home would be a greater concern.)
I would frame up the walls (make sure to seal them to prevent water seepage, humidity, etc. from leaching into the basement). You can insulate as needed for your own comfort. Concrete floors are that big of a problem... I have a concrete floor in my herp "room" and love it - clean up is easy. I'd place my cages about 4-6 inches away from the walls to prevent any water that may find it's way in and down your walls from damaging your caging. Also, if you are able, mount some locking caster wheels on the bottom of the cages to make it easier to move them and to keep them a few extra inches off of the floor.
Not every flood is going to be deep and it really doesn't have to be to cause major damage. Just a couple inches in a persons basement can be very expensive to clean up after. However, if you have your cages away from the walls and on casters those (potential) couple inches of water wouldn't affect your animals as much.
Preparing for the possibility of a flood is a good idea, but find out when the last time your neighborhood was flooded and the extent of the damage that your neighbors faced at that time. Are you close enough to the water that sand bagging was necessary? How deep was the water? Etc.
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Stay United!

"...I say 'apparently' because despite all our progress she is not at all a tame or handle-able snake and gettting her from that cage would re-start a war.. and we've had a good armistice for several months now."-Gus Rentfro - I love this quote!