Should I worry about chain rattle in my E300 dsl Mercedes car.?

These old Mercs are built like panzer tanks and in Europe and the middle/far East they do over 400,000 miles. However, replacing the timing gear/tensioner is not unreasonable. My 1998 E300TD S210 has done 170,000 and purrs like a panzer - a blown glow plug causes a splutter start but that's about it.

Eventually they will get too slack and break the guide that holds them, yes. Also your car will (if it hasn't already) start to run badly, or with less power. Worst case scenario, the chain will break and destroy the timing chain cover.

On the other hand, you might only have a bad chain tensioner.

that's a LOT of miles on an engine and certainly enough miles for a worn timing chain.

That should be looked after. As mentioned above, the chain may still be okay but the chain tensioner could be stuck/faulty.