...well I spoke with an accident repair specialist - someone who I've come across before - and he said at Dutch labour rates he'd be changing at least 5000 euros to fix it.
Hmmmmm
I didn't have to think long and hard about that option!
He was worth talking to though 'cos now I understand why the door rubbers were cut and knackered (so that the doors would fit); why the driver's door was slightly too high and ever so slightly slanted; and why the front wing was shoved up hard against the firewall...
I doubt if I can find a rust free chassis here so I'm now looking into doing the repair myself.
Looking at the parts drawings:-
This construction looks quite simple.
I have loads of experience riveting aircraft structures so for me resistance spot welding seems like a good approach to repair. (This is the reason for this thread - as I don't think anyone has been spot welding here just yet)
So that's all for now folks I've got to go and remove some more bits to get to the spot welds.
I'll update my progress and show some plans for supporting the structure before I drill out the old spot welds and cut out the floor.
EDIT:-
I forgot to say I had previously made loads of measurements of the "straightness" of the chassis =>
How I adjusted the toe in / out, camber and caster on my W123 300D
Everything seemed fine... so despite the aggressive repair it seems that the four corners of the car at the suspension points are all in the right places...