2008 Mercedes Benz R350 R350 water damage problem

I must be perfectly honest with you here. I have worked on a few vehicles that were flooded during Katrina and once a Benz is flooded it will be extremely difficult if not impossible to get it back to normal. The reason is the "brain box" you speak of is one of nearly 70 computers your vehicle has on board. All of the control units that were hit by water will have to be replaced. Also your electrical connectors and wiring harnesses which number in the 100's will now show signs of corrosion because of the water contact. These connectors and wires alone can cause many issues that are nearly impossible to trace simply because current is being inhibited by the corrosion. The first thing I would do is have the computers read by an MB dealer. Have them do a long test (diagnostic test) in which they will try to communicate with all vehicle modules. This test will give you an indication of how bad the damage is. The test more than likely will show several if not most control units with no communication. This means essentially the computers is dead. By starting here you will at least have an idea of what you are getting yourself into. These control modules often times run over $1000 each and knowing if you have 1 or 30 to replace will give you a good idea of what kind of cost will be involved to get the vehicle to a running condition. But keep in mind this vehicle will never be the same no matter how many control units are replaced simply because of the corrosion that will build in the wiring harnesses and connectors.