What to do with the 4.5?
 

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?

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.

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

It may not be as bad as you think
 

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.

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. :(

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