1997 C200 Estate: Steering Arm Replacement

The garage has advised me that the offside "steering arm" on my 1997 C200 estate is exhibiting excessive movement and needs replaced. This
part is what I would have called the track rod. Apparently this part is not available separately and the entire assembly has to be purchased and replaced.
Can anyone kindly tell me:-
1. what is the correct name for this part; and
2. is it easy or difficult to replace?
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