If you're talking about an e220/280/320 don't forget the fuelling/ignition systems are completely different -hence the 300-24valve being "easy" by comparison.
I know that too, but I know this thread is talking about simpler M103 swaps. And my guess of £1000 still stands for that.
But when I get another 190, I'll definitely see if I can get it done for the cost of a donor car plus a day's labour. (I far prefer the W201 to the W124).