Would a compression test rule out the engine itself (ie internal damage/wear)? I just ordered a diesel compression test kit from mercedes source and am not sure if maybe I have the right connector to use that on the gas engine? or maybe the gauge is all wrong and i'd need a whole new gauge for gas engines.

What items with the ignition are you referring to? The owner may better know about specific parts, etc. Also, it would help for me to know what to take a look at.

The owner said it was working fine but then performance changed all of the sudden in one day... I forget what he said, but it seemed like nothing major happened.