tbh I think thats just grime and surface rust rather than any rot worth worrying about. I must admit I don't know if the AMG systems are stainless or not.
No, there is no cat there - it's just a rear silencer and tailpipes.
It would not be a 'straight' swap onto the C240, would probably involve making a sleeve of some kind to accommodate the difference in pipe diameter/bore, possibly moving of the hanging brackets and you'd need to cut your existing back-box pipework (as they're normally in one piece for quite some way)
Nothing a good exhaust fitter couldn't handle though, see Graham's old post from when he fitted an AMG exhaust to his C230K
A very handsome and rare buy for £40, as the seller states, they are nearer £700 new!
You'll never know unless you bid. To be fair, for £40 it'd be well worth your while collecting it, you're only in Cambridge? (that's if you still want one!)