There are two suspension links that connect to the lower part of the rear knuckle. The control arm ( it is the larger of the two ) and the track arm. The track arm runs forward and aft. The control arm is inboard and outboard.
Both are the same as that which is run on the Mercedes 2003 SLK 320. The parts are interchangeable. A brand new lower control arm will cost about $75.It will come with a new inner bushing installed, but the outer bushing which is pressed into the knuckle would still be original. So sounds like you want to change out the outer bushing.
The outer bushing for the control arm cost about $20 each. You seem to think that is the problem . So two of them, one for each side will cost about $40 brand new. Removing the old bushing and installing the new is a job that will more than likely require a press. The outer bushing is pressed into the knuckle. These bushings on the lower control arm are really soft compared to others on the suspension links.
The track bar can be bought brand new for about $34. So one for each side of the car is $68. A new track bar will have new bushings installed. So the old track bar simply needs to be removed and the new installed. That is an easy job.
Very few have found it necessary to change the outer bushing on the rear control arm. You may want to ask your mechanic for his opinion as to whether there really is a problem there which needs correcting.