Hi chaps
Well, I think it would only cost £200-300 to do! All you need is the split folding seats, the shelf and the handle mechanism. I'm talking ebay here, not walking into an MB dealer and asking for all new stuff. I'd only purchase something if it were a good price, good condition, and obviously the same colour/material/spec. In lockdown times, I'm in no rush.
I'm a musician/music producer and often need to carry guitars and keyboards (in cases) and the ability to slide them all the way in would be fantastic.
Yes, I was originally looking for an estate too (I purchased in early 2018), but this car came up locally at a good price, and I went for it. Probably the same situation as you,
@MrGreedy 
The estates at the time were a couple of grand more! They probably still are...
I've spent so much time and money, time and effort sorting out every foible of this particular W212, doing it again is not an idea I relish. They're also getting to the age where they've been abused/remapped, and I know the history (FMBSH) of mine (oil cooler done and sorted) and... well, better the devil you know!
I really don't want to swap my car for an S212, that would be such a sideways move to me. I'd probably sell it and just upgrade to something else. But I really feel no need to - it's a great car and I don't really have plans to sell. I'd rather buy a secondary car for other uses, city driving etc.
It really doesn't look that hard. The harder issue seems to be actually finding the parts as they seem to be rare on eBay!
Cheers,
Ed