Maybe I'm misinformed. My buyer's guide says that the 400E has an M119 engine. So I presumed the 400E had an M119 of smaller displacement than the M119 in the 500E, suggesting that any mechanical differences would be internal to the block. Kind of like the 2.6 and 3.0 liter M103s. Thinking about it further, the 4.2 liter engine in the 400E might be an M116. But I highly doubt the M116 would have lasted long enough to make it through the first year of W210 production.
I'm very confused. Any clarification would be greatly appreciated. But you're right. The price difference between a 400E and a 500E these days doesn't warrant that big a headache.
And 40hp would be greatly appreciated in my M103, but probably less so in a 275hp M11_ 400E engine.