That ugly sludge is keeping the metal in perfect condition! But I hope the engine internals don't look like that.
The alternator needs to rotate reasonably freely on that bolt so as long as the bolt is a good interference fit through the bracket and the alternator lug that shouldn't be a problem, although your rubber grommets don't look very 'Mercedes' (but might be).
Your tensioning arm thing has been cut and welded, but the base looks original, so to fit it properly you probably need the nut/spacer/washers shown in the attached diagram. Based on that diagram it looks like the arm fits onto a stud in the block, is that right? (because the diagram shows a nut not a bolt).
Maybe you could look for a whole new assembly, I think it might be the same as the W108/110/111.