Timing Chain Noise '85 380SL
 

Car has only 87K miles, runs very well and is only used during the summer months. Occasionally (maybe 25% of the time) when starting engine I hear a brief metallic "clack, clack" noise from engine (I assume timing chain). Noise will only last less than 10 seconds. Since the noise does not happen on every start and disappears in seconds, is it a reasonable assumption that it's lubrication related?
I gather from checking other posts, noisy chains are not a surprise on these cars and repair of guides and/or replacement of chain is a major and expensive task. At what mileage do I need to worry about the chain breaking ? Is it ok to just live with the occasional noise at start up unless it gets to the point when it happens at every start and last longer ? Any other warning signs I should be aware of ?

If it's a double row chain, then the chances are that it's not going to break. The guides however can break as they get weak and brittle with age and the stretched chain will slap against them. If they go, the chain can jump.

I'd suggest having it checked out.

At 87K miles it is time for a chain, tensioner and upper guides replacement at least, as the guides get brittle and breakup causing havoc and expensive heads and valves repair.
With that mileage there is chain stretch causing chain slap and camshaft timing misalignment. An average estimate for above job would be in the region of $2300.00 parts + labour with a MB specialist shop. Around $350.00 parts if you can do it yourself.
Or $4000.00 if sprockets and lower guides are also replaced as it involves removing the engine from the car.

Age and heat cycles, not mileage.
Google "Mercedes Preventing Catastrophic V8 Engine Failure - by Kent Bergsma"
Also check the 107 section of benzworld.org. Search there and here for timing chain.

I would not even drive the car until it is checked and corrected. You are describing the fatal timing chain noise.

you have the warning signs - you wont get many more that will be of any use ... as in engine failure = too late :(
At the very least get it checked out , or check the chain stretch yourself. DIY in the quick link section