300-24, saloon and coupe, E300 24 valve diesel estate
I would personally change the timing chain at 100K miles or when it gets noisy whichever comes first. Things to look for. Does the chain sit well on top of the cam shaft sprocket without undue lateral movement. Does the sprocket have any wear on the front (left hand side looking from the front of the car) of the teeth? Can you lift the chain slightly on top of the sprocket? Do the chain rollers look and feel lose?
Is it a single or duplex chain.
For single chain I would be looking to do this at 120k or if there is noticible wear, the duplex chain will last a lot longer.
A rule of thumb is that if the cam lags the crank by 5 degrees or more then change the chain.