This is an awesome discussion. Great input, and from knowledgeable owners who can walk the talk.
Since starting this thread, I asked both of my longtime indy techs this very question, and one said the 1986-1992 300E and the other said the same vintage 260E. Apparently the variable valve timing on the newer sixes makes these engines slightly more complex with a hair more maintenance requirements and expense, although the 93-95 straight 6's are still quite good. They said the 400E is also very good for a V8, but being a V8, it does end up costing quite a bit more to maintain in the long-run. Not less reliable or durable per se, just more engine to keep up, especially the 2nd head, of course.
On the 300E/260E's, they both said all you really need to do is rebuild the head every 150-250K miles and the bottom end will last longer than you care to keep the car...with proper routine preventative maintenance, of course. And in the end, the straight 6's will probably give you the lowest cost per mile in maintenance over long-term ownership.
One of my independent shops has been working on my cars for 25 years and the two partners are absolutely the best mechanics I've found anywhere. So I give their opinions a lot of weight. As much as I REALLY want a 400E, I might just consider a well cared-for 86-92 straight 6, since I am after absolute reliability and durability. After all, I am really a Benz diesel head used to 400K+ mile engine life, now looking to dip a toe back into gassers...
