Provided the oil was changed, the timing chain on an M104 should be good for a very very long time. When it rolled over 200k it may be worth instpecting for stretch and inspecting the guides. But I wouldn't expect to replace it until the 300k range.
The key to it is maintaince. MB specs out Mobil 1 0w-40, use a high quality synthetic oil and change it every 5k miles.
The 606 diesel which has a similer chain and cam set up to the M104 seems to create about 1 degree of chain stretch every 100k miles. Provided synthetic oil is used of course. I don't know what the stretch limits are on the M104 but I bet it is 3-5 degrees.
Purchase a FSM and read up on it.