I've had racks go bad in two cars I've owned over the last 10-15 years. Both presented the same symptom-- random "tightness" or resistance to turning that came on suddenly. It was not repeatable; if you let up on the steering (or bumped the wheel the other way) then pushed in the desired direction once again, it would usually cooperate.. I can't recall if it was problematic turning only one direction, but it made for interesting driving when your steering wheel freezes up in the middle of a turn!
Does your Mercedes require a special fluid, and if so, has that fluid been used religiously with no top-up of regular fluid? That's how the rack in my VW got torched, I used regular Autozone power steering fluid for top-up instead of the uber-expensive synthetic German fluorescent green Pentosin stuff I was supposed to use. Turned all the fluid into a tan milkshake color and trashed the rack. After a flush I was still able to drive it but the problem never went away until I replaced the rack.