What an epic thread, just read it from start to finish, congrats to you Guy!
MY GTR's got 14k miles and I'll probably put around 8-10k a year on to reduce the mileage going on the Audis. That said and in reference to high performance cars and costs, my (6) RS6's have all been sold with a minimum 70k miles on and the last V10 went with 188k. All have been mapped / fettled and all got driven hard.
Never had a problem with bodywork however and never really relgiously washed them either. I put this down to road salt used in the winter, and me piling on 35-45k a year in miles; they haven't had time to rust!
I seriously can't imagine putting more than 10k a year on my GTR, simply not comfortable enough for 1000 miles drives in one hit!