I think that it doesn't matter whether the engine would or would not fit under the hood...
Vlad, as you already wrote yourself: you have seen discussions about engine swaps in the older models and that is just the key-word: older.
If you want to change a current petrol Benz into a diesel, you need to change a lot such as the wire harness, the computer system, the dash cluster, probably the cruise control, your AC will become a pain in the a$$, your automatic transmission will get communication problems, your ABS may become inoperative...
Mercedes has changed their cars too much to even consider an engine change like that!

I think that it will be a LOT cheaper to import an E200CDI, E220CDI, E270CDI or E320CDI. Importing a car like that with the help of a RI will be expensive as well, but will end up a lot cheaper in time...

...just my opinion...

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)