A compression test will be usefull to find whether there is a really defective cylinder. Unless you are very familiar with your gauge and an engine in particular it is very unlikely that you can tell if an engine is uniformly worn out or not. So many things vary the actual values.
Normally MB diesels have compression readings around 300-350psi. The gauge I use has always been higher than that around 350 -400psi. A cylinder running 250 with others running 300 would bother me.
After years of seeing diesels wearing out I have decided that the best gauge of future engine life is how much oil one uses and how much trouble it is to start the car when its 30 degrees out. If the motor is wearing out evenly it has a good future if it is using less than a quart every 1000 miles. At a quart to 800 miles I start looking for reasons. At below a quart to 500 miles the car will start poorly when cold and be a real ***** if it doesn't start in the first attempts.
Bosch Master, ASE Master, L1
33 years MB technician