I may be missing the point here, but if your mechanic says the joints on the idler arm (steering arm same one bottom of steering column) will pass MOT then there cant be much play in it, I would not change it. If you replace with a used one it will be no different, and the difference between this and a new one will be too small to notice. Sounds like a waste of time and money to me, though I must say these parts (SLK320 W170) are surprisingly cheap but if you do change it you may as well do them both buy new. Good luck