Dunno about that, OM606 is pretty long and heavy; they suck to work on too, esp after the return lines start to leak fuel into the injector wells or when you have to pull the intake manifold off to do almost anything on the LH side of the motor. Don't think there would be much that is "easy" about dropping one of those in... 4cyl VW would not be bad, there was a guy on TB that put an AHU TDI into a 245 and I don't think it was a big deal, just not a bolt-in, since BH is different. I think he used a Toyota trans and an adapter.
An Audi five would not be bad, back of it should bolt together, parts in front probably bolt on to the block and head, suspect mounts would be only big difference. Euro 2.5 TDI would probably be most straightforward way to get something more modern and capable of higher output in there, structurally very similar to the older six.
Of course a D24T would be the easiest of all, cheapest, and nicest to listen to... way better fit than a POS Benz lump, and for what it would cost to even get a stock 606 in there, you could have a D24 turned up more than enough to outrun it anyway...
