I'm afraid I can't answer all of your questions but I thought I'd chime in since I own a '92 190E 2.3. 108,000 miles seems to be pretty early for a timing chain to be going on an M102 motor. It's a double row chain and it's pretty short, so it doesn't stretch as quickly as some of the longer chains, especially the single row chains. The timing chain tensioner could also be the culprit for this noise if it is worn. As always, a lot depends on how well the previous owner performed the required maintenance.