I am not fully familiar with the 79 300SD body style, but Mercedes designed their steering assemblies about the same during the 70's and 80's
The design is thus.
From the left wheel, drivers side you have a TIE ROD that has BALL JOINTS on each end. This TIE ROD connects to the STEERING KNUCKLE on the left wheel to the PITMAN ARM that is attached to the STEERING BOX. The PITMAN ARM is then connected to the DRAG LINK. The DRAG LINK transfers the steering input to the right hand wheel by being connected to the INTERMEDIATE STEERING ARM. This arm is what is connected to the IDLE ARM. The IDLE ARM KIT is what you are showing in the picture above. The IDLE ARM BASE is welded to the frame of the car. It allows the DRAG LINK and INTERMADIATE ARM to transfer the steering input in the correct geometry to the right side TIE ROD. The right side TIE ROD is connected to the RIGHT TIRE STEERING KNUCKLE
Additionally attached to the drag link is a STEERING DAMPER, that dampens any road inputs to the steering assembly.
What usually needs replacing are;
The tie rod ball joints. It is cheaper to buy the complete tie rod assembly, 2 ball joints and the adjusting sleeve, as one piece then to buy them seperately.
The drag link due to the drag link ball joints wearing out.
The idle arm assembly. You replace the rubber bushings, bolts and washers. The base is welded onthe frame
The steering damper. Its a shock absorber and it can wear out.
It is RARE to replace the the pitman arm or the intermediate arm on a car. It is usually replace if damaged in an accident. It can of course rust and need replacing but the body will be a complete mess by the time the arms rust out.
The play in the intermediate arm is probably being caused by the wear of the rubber bushings and not the arm. Buy the kit and once you disassemble the idle arm you will be able to see if the arm is bored out.
Here is my link to the 123 82 300TD suspension rebuilt.
300TD suspension rebuild progress