Potomac German Auto has a unit on ebay for @ $200 I've bought from them & they seem to know their stuff.
Replacing major SLS components with new @ dealer prices parts makes little sense - to me at least. Not when Bilsteins with H&R lowering springs all around can be had for less than a pair of hydraulic struts.

There's no market differences between a 420 & 560's that I've noticed, despite the better engine too. So I'm not that inclined to preserve a $3K rear(only) suspension setup.