I'm partial to the M104 powered 124 chassis.
I find that the car is perfectly sized. It is still within 1/2" of the W210 chassis in dimensions. The back seat is also big enough for 2 adults to ride comfortably.
My dad had a 380SE, and the short wheelbase W126 cars have a shamefully small back seat for the size of the car.
With the 3.2 liter M104 engine, a new engine wiring harness, and the newer head gasket that doesn't leak, the W124 chassis car is about as good as they get, IMHO.
What's more, they look great with an OEM wood / leather steering wheel, chrome shift surround, and S600 shift knob: