The actual parcel shelf part is no longer available and hasn't been for some years, it was a single part that made up the rear seat upright and parcel shelf and was circa £1000 10 years ago when they were available.
However there is a repair panel available for the base of the rear windscreen from mercedes. It wasn't expensive when I last bought one about 3 years ago.
As for wings, simply change the fronts for new, the rears have repair panels available also. These were circa £300 a side from mercedes 3 years ago.
Remember that 560 front wings differ from 420's and 500's in so much as they have a slightly wider arch.
If the rear screen is good then keep it but renew the rear rubber and trim when you get the rust repaired.