One of the main problems is the upper end. It probably needs new valve guides, and at that point, if you remove the heads, it's probably best to do a valve job. The problem with that, besides the considerable cost is that the lower end is probably worn out. It smokes considerably upon rapid deceleration from high speed and smokes in stop and go traffic upon adding foot to the gas pedal.
Upper end issues like worn valve stem seals cause hideous clouds of smoke upon start up, the likes of which i've never seen before.
I suppose it's possible to replace valve stems (if that alone would cut the smoke-starts) and just live with the other things. We all know the M117's are super tough and it would soldier on for a while. It runs great on the highway.

Okay, so lets assume on a 30 year old car with very possibly the original engine with signs of wear on that engine of a serious nature, we can guess that the valve guides are likely shot.
If replacement of stem seals stops the smoking, how long is that repair likely to last combined with worn out guides?

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63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6

New valve seals may not last long at all. The slop in the worn valve guides may cause the seals to wear out quickly - as soon as 10,000 miles.

It sounds like you're wrestling with the fact that it's time to rebuild the whole engine.

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95 E320 Cabriolet, 159K

Ya, actually wrestling with letting the car go.
Thanks for the reply
I've got the other two to devote attention to.

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63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6

Going back about 18 years, my 350SLC (3.5 litre version of your engine) was using a lot of oil (1 litre every few hundred kilometres). It was smokey (blue smoke) at idle, after trailing the throttle and after starting off. New valve stem seals cured the problem... for about two weeks. I then removed the heads and had new guides fitted (as well as four new valves) and another set of seals. That totally cured the problem. While the heads were off I noticed that the bores still looked good with obvious cross hatching still evident, despite already having high kilometres. The iron block M116 and M117 have a reputation of longevity in the bottom end, although I believe rings eventually cause problems on very high milage examples. Since then it has only travelled about another 130,000km and only now is it starting to show a hint of oil smoke after extended idling, probably due to the valve stem seals ageing again (it has been 18 years). A set of stem seals would probably confirm whether or not your rings are OK, but as was the case with mine, I expect they would not last long due to wear in the guides. I expect that a top end overhaul will fix yours unless it has very high milage or has had poor attention to oil changes. I must admit I have been very fussy with oil chages. Since it does not cover great distances it gets fresh oil every 2500km (oil is cheap, engines are expensive) and a filter every 5000km. Recently when adjusting valve clearances I was impressed with just how clean it is inside, despite having covered around 330,000km.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.

Greg,
Thanks for the advice. Unfortunately, it wouldn't be worth pulling the heads on this car. The whole thing is very worn out in every way. Pulling the head means replacing a lot of parts, and I'm not sure how bad is the lower end. Stem seals might last for a year or so, but maybe not.
I've got to focus on the 220S and 300D.
I'll try to find a good home for the 4.5 though. Either a motivated DIY'er or a Mercedes dismantler that'll make use of it and not crush it.

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63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6

joshhol,

Seems a wet and dry compression check and leak down test might give a good basis for pronouncing the lower end flogged or healthy. If the car is otherwise in decent shape, head work may be all that is necessary to make it a good drive again. Good luck, Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)