You want Bolt Pack 1 from here:
Packs of Wheel Nuts and Wheel Bolts from Alloy Wheels Direct
Set of 20 bolts (5 per wheel for 4 wheels) RAD part number: 4143 Shaft Length/mm: 39 Thread pitch: M12x1.5 Drive type: 17mm Seat type: 12mm radius Fitment: Mercedes 107/124/126/129/168/170/171/201/202 203/208/209/210/414
Note:
I am fairly certain you want Bolt Pack 1 because that is the same kit for both the 124 and the SLK (You are lucky !)