Possible flood water problem!

[joe88moore]

Hi, I bought my 2005 a6 automatic 3.0tdi v6 Quattro from a friend the other week. It’s a lovely car, been owned by an Audi mechanic for years, he has had some work done to it in that time - stage two map, upgraded intercoolers, so she is a beast, but in my lovely countryside area, I had some flood water. I was following a lorry through some the other day, in which I was driving in its “wake” so keeping my car quite dry. It then decided to stop as soon as it got out the flood water, and I was sat there, so obviously went from second gear down to first in drive. The lorry sat there for a moment and then carried on. After I came out of the water my car judders and shakes in low revs in the higher gears. I’ve found that when the car changes gear from (mainly) 4th to 5th it hates it, and have found that if I want to drive the car at the moment I need to use the + - on the shifter and keep it at fairly high revs. 
My friend who is the Audi mechanic has said that he doesn’t think the car will survive a gearbox fluid change, and it does sound like it’s taken some water into it. 
Any thoughts from anyone here?

thanks