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?

__________________
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