Before considering a rebuild, measure compression pressure in each cylinder. If >350 psig, don't rebuild. Even if low, it could be due to valves (cheaper head job), and a leak-down test determines that. Once you tear it open, you can't test.

Main and rod bearings can be changed with the head on. In most old U.S. cars, you can do in the car, but the OM617 must be removed to take off the upper oil pan. That is the main thing that restores oil pressure. But, the low-end rarely wears and bad main bearings would give a low clunky sound.

Your symptoms sound like the oil pump chain tensioner failed, which would cause a loose chain and slapping sound. It did on my 1984 300D. I changed it thru the lower oil pan (engine in car). The part is <$10 here at P-P. Read my post w/ photos since tricky to remove the sprocket bolt but can be done. Others claim you must remove the upper oil pan (big job, engine out). You must remove the oil pump, so replace the O-rings on the outlet tube (Viton is best, and no you don't need a dealer for O-rings). You also must remove the #1 bearing cap, which lets you inspect bearing wear. Might combine w/ a front seal change if needed. I don't recall that I did, but might help (get more fingers in there).

When you do actually need a rebuilt, search for my post. I took my 1985 block to the "new sleeves" level, but haven't taken it to a shop to deck and hone to the pistons. Another guy posted many photos. People thought he was crazy, but he showed photos where he has a full machine shop in some remote ranch, and decked the block etc. I noticed that rebuilt OM617.952 engines appear to have dropped from $8500 to $6500 (ebay), but still out of my reach. There is also a set of new turbo pistons on ebay for ~$1200, which seems a deal since 1 new piston was $550 and you can't even find them. Someday, a custom piston maker like Egge might find the market, or someone here might front the NRE cost to get some made and hope they can resell them. Even cylinder sleeves are getting expensive and rare.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans