Yeah, I don't count the wiring harness in any car when considering the engine. However, I have heard of M104 owners having multiple head gasket replacement episodes.
In the grand scheme of things, the M111, M103 and M119 are "no brainer" buying decisions. While the M104, M112, and M113 are great engines too, there is some serious investigation than must go on. Gaskets for the M104 and crank balancers are M112/M113 issues. However, at least a leaky head gasket on the M104 dosen't come apart and potentially wreck thousands of dollars of other stuff.
For a buy it, drive it, forget about it, kind of experience the M111 is near the top of the list, but has the flaw of not propelling anything larger than a C-Class with much verve. However, where the larger cars are sold with M111's, gasoline tends to be pretty expensive. The M111 (aspro) is very frugal. I often get 6.5L/100km's in the C230 in highway driving. That's phenomenal for the level of equipment that I'm hauling around. Climate control, heated seats, sunroof, four airbags, super-stiff structure, and so on.
As to the street cred of the M119, look no further than the W124 E500 cars. I challenge anyone to find another marque that offers similar performance with the kind of durability and longevity that the E500 offers. M5? Hahahaha. Race horse manners, sure, but race-horse expenses too. M-series engines make tons of power, but have very expensive M-specific bits and parts costs are closer to Ferrari than Porsche.
As to the M103, take a drive down the freeway sometime. Notice how many 86-92 300E's there are? Even in "Mercedes rare" Edmonton, they're all over the place. They just go, and go, and go. The M103 in our old 190E 2.6 (Dad still has it) is terrific. Fifteen years and over 200K-miles, and it's still tight and strong. Top end? Not yet. Bottom end? Probably never.
However, the M104 crowds the podium with the M103. Just make sure the one you're looking at has been sorted out.
1998 C230 "Black Betty"