Hi Joe - I recognise you and the car from the Facebook pages.
I'm also driving an MB that previously belonged to 2 of my Grandads, both of whom have passed now, so I know sometimes it isn't just as simple as 'scrap it' without even trying to put effort in! Not to mention you've got a nice clean MB6 there (with the bumpers on ) which is getting to be a seriously, seriously rare car.
Not sure if you remember me but mine needed a LOT of welding doing to pass the MOT last November, I ended up having both lower sills completely chopped out and replaced as well as patches in the rear wheel arches, an area behind the ECU in the passenger footwell, and a couple little areas on the outer sill (where I can see you've got big holes) right at the back, and the opposite side to that I had welded 18 months before because the jacking point collapsed. All in it took round about 4 days for the guy to do it all and cost me £600.
I think maybe the undersealing/protection following the last bit of work accelerated the rot in the 18 months between MOT'S, but now I've got all new metal down there I'm quite hopeful that it won't return anytime soon. When we get any sustained good weather I'm going to get the sill covers off and neutralise any surface rust that's started on the outer.
So basically I think yours can be saved, it just won't be too cheap! There's a few more tricky areas to sort. And wherever possible with the lower sills, you should get them cut out and swapped for new metal, or the remaining areas might go eventually. You can buy replacement outer/side sill pieces to weld in, I discovered these a few days too late to utilise them on mine, but mine was mainly the lower sills anyway.
It's good that you've stripped the car out, so any more problem areas you'll be able to find and won't get a nasty surprise. And it's good that you're not in a rush to get the work done, and have somewhere you can keep it off the road. It means you don't have to try and get it rushed to pass an MOT or something.
Oh and another option for smaller areas is to ask anybody scrapping clean shells to cut bits out for you, but I wouldn't hold much hope for that.
Just my experience anyway pal, good luck with the restoration!