The biggest problem is that the sump on the MB is in front and the sump on the Chevy is in the back. This means that the crossmember has to be removed to put the Chevy engine in the MB, and that you then have to find a way to put a suspension back in. Biggo Problemo.
The usual "fix" is to stuff a Chevy front end from a midsized RWD car under the Benz, totaly ruining the handling, etc. More trouble. The fancy fix is to fabricate an offset crossmember so the oil pan can stick down were the crossmember should be.
The repair for the broken chain is far easier than the hassle of getting the chevy engine in there. You will only be out the valves on the right head, a head gasket, a chain, chain guides, and some time. You will probably need to pull the engine (vastly easier to remove the heads with the engine out!) and the left head needs come off if the chain was dropped and you can't fish the replacement through (chain cover goes under the heads, a royal pain to re-install if the heads are on). If you do it yourself, you won't be out more than $2000 for parts at the most. Costs $4000 for Hans to do it at $60/hr.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!