Ah yes, the ol' timing chain debate!

There are many different MB engines and timing chain arrangements. The 123 diesels are some of the best setups in this regard. As important as the chain(a double roller thank goodness), maybe even more important is the tensioner. Without tension on the chain, it will flop around and break a guide or something resulting in catastrophic failure. That said, the tensioner itself is simply a piston in a bore that holds hydraulic pressure generated by the oiling system. The good news is that this piston does not move rapidly inside the bore meaning that there is little or no wear.

There is a school of thought in which people feel that they must change the tensioner any time they change the chain, and who am I to argue in spite of the fact that I don't understand how it would wear out. In a high mileage engine with low oil pressure it is easy to understand that it might not hold the chain tight, particularly during start up before oil pressure is developed.

As far as when to change the chain, it's sort of like asking "how long is a rope?". An engine that has had religious, frequent oil changes, does not experience much particulate matter circulating with the oil. In that case the rollers and pins wear very little if any.

I think that IF the engine still holds very good oil pressure AND if the chain shows little stretch, you are okay.

To check stretch accurately, you need a dial indicator and the MB procedure to know exact valve lift specifications for accurate measurement.

You can get an idea of chain stretch by removing the valve cover and rotating the engine by hand using a 27MM socket on the crankshaft snout. Watch the mark on the number one cam bearing stand that is about 45 degrees from the top toward the drivers side. Once the camshaft mark perfectly aligns with the cam bearing stand mark, stop and look at the harmonic balancer timing mark. If the crankshaft is more than a degree or two past TDC, you might want to think about measuring it more accurately with a dial indicator, or changing the chain. If you change the chain, inspect the guides that you can see for damage or wear.

Good luck,