Ours was done by a body shop owned by the supplying dealer: it was in Wilmslow, and cost £1000 in December 1999. It was a lot cheaper then using Merc bits for everything: they made up up the seat and back rest, using merc bits for the rest. Esentially it was a simple job, since the body shell was not modified. I think the group was called the Pendragon Group: they are no longer merc franchisees since MB took over the retail a few years ago.
1999 E300Td Elegance Estate, 1993 500 SE, 1995 Citroen Xantia 1.9 td, 1997 Land Rover Discovery tdi
At the time we were quoted £1700 for a full MB parts conversion. The saving was said to be from using a locally made non MB seat squab and backrest. We might have been taken for a ride!! I have done a similar job on a Citroen CX estate, and the body shell already had all the screw parts and so on fixed. The MB seems to be the same. The job is pretty straight forward: no welding or cutting etc. Just get the bits and fit them. Good luck!!!