If you can arrange it, adjust the valves again and then do a leakdown test with the compression check. And be there for the test.

Like TxBill and LarryBible say, leaking from the bottom (past the piston rings) isn't cheap but a diesel engine can handle it for a long time. You'll need an entire engine sooner or later. Decide to fix or roll the dice again with another car.

If you're not leaking from the bottom then you're in luck. Quality valve jobs cost half of a new engine. Decide to fix or roll the dice again with another car.

That "smoke" may just be partially burned fuel. 140k is not a lot of miles for this engine. One of the tricks is to only fill engine oil to halfway between the marks on the dipstick, because of the goofy rear crankshaft seal. Oil above halfway tends to run out there. A cheap and easy check as it makes a mess. If that's the case then it's on to fuel and driving habit issues.

The buzzing clutch may not be the clutch. You're going to have to find a real good, experienced mechanic to find the real cause.

Finally: is a 19yr old car really worth the frustration if you can't work on it? Things wear out and need fixing. I became my elderly aunt's favorite nephew again after I fixed her Caddy a few years back. Had a buzz and vibration. Two dealers couldn't find it. Took 30min to diagnose and replace a u-joint. Not being able to work on a car is not a strike against the owner. Or against a shop (how many rear wheel drive independent suspension MB diesel cars do you see on the road today?). Only lucky collectors get their money back out of a car, and not with these cars. The rest of us use them for jobs, transportation to other stuff, or for the pleasure of the car itself. Only you can decide where the buck stops.

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daBenz - 1970 220D