The 190E 2.6 motor is more or less identical to the 300E 3.0l motor. The difference of course is the displacement and power.
The M103 motors in both these cars don't experience the head gasket issues like the M104 motors do (C280, 300CE 1990+, 300E 1993 etc.)
The M103 motors are pretty bullet proof.
Any of those three mentioned cars would be a good choice, although I suspect the 400SEL is going to cost more to maintain, while the 190 would cost the least.