David if it were me i would get it dragged to a good bodyshop sooner rather than later as leaving the rust to eat any longer is going to kill some of the rear parts of it and make it a parts only project
As it is i see it being at least £600 for the rust bits to be cut out if you can find other wings for it that would bring it down a bit but to get the stuff cut out, good stuff welded in and done in a good job is going to be a good few days work for someone to do and it does run as well and sounded nice
That green estate you saw him in was my one as well, He was trying out the cruise control and some other work he had done on her