Don't get me wrong, it would never hurt to change the chain for preventive maintenance reasons. However, it IS possible for the chain to have such high mileage and not be stretched. For this to be the case though, the oil has to be changed often and religiously. By changing oil VERY frequently, you prevent particulate from wearing the chain.
The standard MB recommended interval will usually not provide a nonstretched timing chain at this mileage. But oil change mania will most likely reward you with a chain that has zero stretch at this mileage. Been there, done that.
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in