You really should find an independent shop....One that is Proficient with Mercedes and particularly your car model... They don't call them stealerships for nothing. As for the Compression/Leak-down you can do this as a DIY if you are handy. Testers are pretty cheap and even cheaper from Harbor freight. As for the valves... I dont know, I would do them both if it was me, Balance and both sides wearing the same would be the benefits of that, I dont know if you can do just one side or even just the one cyl. it's more of 'While you are here, you might as well' sort of thing..

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed