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.
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.