I agree that the M111 is surely the best choice if you want to "buy it, drive it, and forget about it", but here in the U.S., it was only offered in the C class.
According to MB Doc, Benzmac, et. al., the head gaskets for the M104 were redesigned multiple times, maybe as much as a dozen times, but the most recent re-design is said to be leak proof. My Mercedes mechanic says he hasn't head one leak, yet.
That said, in an E class chassis (W124 or W210) with a six-cylinder engine, I'm partial to the M104 solely for the added power, but admit that the M103 is every bit as smooth and refined, and equally long-lived. Everyone has seen or heard of M103's with 300,000 or more miles, but the M104's are so new, it is rare to even hear of one with over 200,000 miles. I known they're out there, though.
If it were mine choice, though, I'd get the W124 chassis with the 4.2 liter M119 engine (1992-1995), or the W210 chassis with the M119 (1997 E420 only).
It's hard to believe that the difference in fuel economy in the W124 chassis from the 3.2 liter M104 to the 4.2 liter M119 engine is only 1 mpg. The 1997 E420 gets even better gas mileage due to the 5 speed transmission, and possible upgrades to the fuel management system.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".