Do your own second opinion. They need checked that all or any individual valve clearance is not gone. Not set to actual recommended value initially. Just pull the valve cover and get a set of feeler gauges. Follow instructions in the archives.

You can impoverish yourself quite easily by paying for all services on an older car. It's bad enough costwise on a much newer car to use garages .

These older mercedes diesels are great learning or self teaching vehicles. This site is all you need to get started. Add tools as needed over time if you have none.