A couple of thoughts.

First, since your friend does not know much about these cars, but wants to make sure it's is tip top shape I would do the following.

Ask around for the best independent MB shop in town. Take it there, and pay the shop $ 125 or so, to have it examined. It's called a Pre-Purchase inspection, only he already bought the car.

You sure don't want to start replacing things that may have been replaced recently by the former owner.

By the way, the rough idle could be the motor mounts. If the belt tensioner has never been replaced, then it is about due. The replacement of injectors at 157K sounds premature.

I just had the timing chain done on my '92 300E M103 at 218,000 miles. I was having the front engine cover resealed, so did the chain. Mechanic said it had not stretched and could have gone on and on. Mobil 1 every 5 k.

Stev