Clicking on cold start and an oil leak near the timing chain cover? I'm no expert on timing chain life on an M103 engine. I never had any problem on my 87 300E in 105K miles, but I would immediately check to see when and if the chain tensioner and chain have been replaced. Time would certainly suggest a tensioner replacement and well over 100K miles then the chain is suspect.
After the car has sat for 24-48hrs, open the hood, have some one crank the car and listen for the chain to hit the cover. My MB Dealer calls it "the death rattle". If the chain goes, the valves and pistons kiss and tell, so this is not something you want to ignore.