An M104 definitely has a timing chain. Maybe two.
The timing chain keeps in synch the camshafts and the sprockets. Well, maybe not quite in synch. The camshafts make one rotation for every two crankshaft rotations. The timing chain along with the sprockets are the mechanical link between the crankshaft and the camshafts. A two-to-one ratio of camshaft sprocket teeth to crankshaft sprocket teeth ensures that the camshafts are in phase with engine cycles.
There isn't a factory recommendation on when the timing chain should be replaced. Anytime you do work on the head, is probably a safe bet. Some might say every 100-150K miles, but that's probably for engines older than the M104.
I say there might be two timing chains because some MB engines have a second timing chain off the crankshaft that turns the oil pump.
Hope that helps.