The M103 is a pretty loose fit in a W126 so it's about the easiest modern MB to maintain. The very similar M104 is a more snug fit in a W124 but still not difficult. The W124 AC system is not as robust or easy to service. You don't hear as much about replacing W126 evaporators as you do W124 evaporators. And I believe that's a pull-the-dash job. There are trades to consider such as easy to replace motor mounts in a W126 vs easy to replace front rotors in a W124.
The W124 will get decent mileage where I think the 300SE/L gets pathetic fuel mileage. A 4 liter Lexus V8 gets better mileage.
The trade-off is between space and driveability. Any W124 is a sports car compared to the W126, but the back seat of a W124 doesn't give me a sense of luxury.