Hi Norry, I have a MY98 Uk turbo wagon, it had the same rear arch and sill rust issue's and while it did cost a few quid to put right it was well worth it. My wagon is pretty much rust free and drives as good, if not better than when I bought it 5 years ago at 94000 miles. Having now more than doubled that mileage at 218000 miles it's not worth getting rid of, plenty of mileage in it yet, even after an new ecu and remap and supporting mods to up the power.
As has been said, you could flog the parts, but once the interesting parts have gone, the carcass may take longer to sell, and in the meantime you have no motor? I would say keep it, if it's not broke why fix it?
I want to get to 250,000 and then think about a rebuild with stronger internals, so I'm saving up for the next 18 months to 2 years to pay for that..these classics just run and run...
Cheers,
Col