There is a strange thing with the 94-95 E320 and E420 cars. Usually, I would always recommend the smaller engine if you're interested in lower operating costs. This situation is an exception.
Thanks to VERY tall final drive gearing, the E420 hasn't much of a fuel economy penalty compared to the E320. However, that gearing also steals some performance. If you're inclined, a change of the final drive ratio gives near-E500 performance, but with a resulting economy loss.
The M119 is very durable with only a few easy solved problems. Oil feed tubes in the heads are easily replaced. The V-8 cars have looooong timing chains that have to be changed more often than the short six cylinder ones, but the M119 is nothing like the M116 for chain problems.
The E420 sold for a hefty premium over the E320 when new (in Canada I remember is was over $10K difference) but in the used market there is little difference in pricing for a clean "last of" the W124's.
The M104 suffers from the same issues as the M119 in the chassis department (wiring harness) but has the added worry of the head gasket. There is some talk that once the "updated" M104 gasket is installed, it's fixed, but some people seem to be replacing head gaskets on a regular basis.
What I would recommend is simply finding the best car you can, and buying it. Also, don't pay too much attention to very low mileage. These cars don't like to sit idle. Don't pay extra for low miles.
1998 C230 "Black Betty"