The worry about re-shells is a recent thing as certain Minis like the Cooper S became much more highly valued and collectable.......many of them would not be around today if they had not been re-shelled in the 1970's and 80's when it was not worth repairing an original shell, in the days when you could buy a good solid 'donor' car like a low mileage 850 Automatic Mk1/2/3 instead. People would have thought you mad if you went to the lengths of panel replacement seen today on early Mini's to retain an original shell!
I know many will cringe on reading this.
The last Cooper S I had, an Australian 1970 Model, and even rarer as it had the 'flappy' door handles, of which they only made something like 127 of and further rarer as it was an ex-Highway Patrol Car, had rusty sills and a rusty floor. I recall paying in the order of $1800.00 for it, which at the time, was a fair price.
In the early 80's when I bought the car, it was so comparatively expensive to carry out such repairs, that these panels were simply not available and rust to that extent was considered a death sentence. I stripped it out for the parts and cut the shell up.
Oh well, easy come, easy go.