The 90+'s 300E are not as fast as the earlier ones because they have a lot more standard equipments. They have the 177hp M103 motor. 12 Valves - 6 cylinders.
A/C Evaporator as you have pointed out might be expensive if they need to be replaced in a 124. The dash needs to be removed I'm sure you know that.
The 3.2l I-6 M104's (24 Valves) make a good 210+ hp I think. These are in the 93+ cars. They are known for head gasket problems.
I heard the 4.2's don't have this head gasket problem but the timing chain needs to be done at around 100k or something like that.
I'd get the E420 just for the V-8. Just make sure you get a complete Pre-Purchase Inspection from a dealer or something.
However, I can see why you would like an I-6 better for maintenance reasons.
The 4Matic model - again I never owned one.. so just from reading stuff up - could be very expensive if it breaks (the 4wd system)
I rarely see a rusty W124 even up here in Boston.... so it seems that they are pretty good at that.
2008 BMW 335i Coupe