Going back to the rust and restoration, you've got to remember that the sills are generally the first area to rot anyway, so that will be the same with a restored car, plus 10 years is a long time, I'm sure out of the factory they would have been rotting well be then. Sometimes we forget the timescales we're talking about, especially if the car continues in daily use, no reason it shouldn't rot at roughly the same speed it would have from new.
Of course a much better than factory rust protection scheme will help, plenty of paint and cavity wax, pretty much the same reason why there are some original un-welded cars about.
Anyway I've discovered a new preservation system, taken from one of our other threads, apparently the best way to preserve an old car is to bury it in silt at the bottom of a freshwater lake
In terms of the insurance, you really don't want them to write it off, regardless whether you get the salvage or not, so if you can come to some deal for them to pay for repairs based on used parts that would be best, otherwise you end up in the situation one of our other members (Phil ?) was when his was crashed and they wrote it off without telling him, search the forum for his story.
To be honest I probably wouldn't put a claim in for this, I know I'd end up paying it back in increased premiums, and a bonnet some paint and a play about with the roof wouldn't be worth it to me, but everybody's circumstances are different.