Mercedes cams can be broken during install. I remember the mercedes literature stating that an even tightening of the 104 hollow camshafts was critical as the cams are "very breakable". I remember b/c the wording struck me as odd. I've never seen one break on install but I'm really careful to pull them down evenly.

When setting the cam timing you bring it up to TDC and then align the marks on the cams making sure the intake cam is fully counter clockwise in the advancer. Disassemble the tensioner completely. Install the outer housing first, then the plunger, followed by the spring, damper, and cap.

FWIW, I thought that engine was an interference engine, being based off of the 104 which is an interference engine. If while turning the cams to align them you encounter a firm resistance back them up and turn the crank about 30 degrees. Proceed turning the cam until the lifted valves seat and set the engine back to TDC and continue from there