i did all four corners last year shocks and springs.
I used sachs from euro car parts, i waited for the 50% discount days then bought them.
If your only changing the shocks this maybe the reason why your having issues, my car was boat like, and sitting very low. once i changed the springs and shocks the car tightened up and sits at the correct height (well no more crawing over speed humps now).
The rear springs will sag especially as the mw is a heavy car and there is a lot of weight swung over the rear axle, if the springs are this bad the shocks wont make an difference as the car will be sitting low and when you hit bumps it will hit the stops making it feel very hard (mine did this).