I would say, don't take a first year, 2003. Not necessarily the FL, but not the early models. Avoid 2003, they are the most likey to have residual troubles.
There was a rework of the interior with increased finition in mid-2004. Not a face-lift, no exterior modifications, no mention of that anywhere. You can see it easily with the plastic base of the front seats, it is much better after the rework, less shiny and more expensive-looking. And some cars before this suffered from a problem at the factory, making the wood panel in the passenger side not matching the wood line in the door, not very elegant. Could be repaired by the dealer.
This reworked model would be a good buy, because they also reworked some electronic functions. But even a not reworked 2004 is good, I only would try to avoid the 2003.
One thing is absolutely imperative: you MUST check the past of the car to see if the recall was applied to it. Do not buy a car that wasn't recalled/no trace of this recall.
This recall was massive. Especially they changed totally the battery-gestion device, the SBC was revised, and a lot of electronic features were updated. Even the wipers were modified! You almost can say that not-recalled W211 are Beta-cars, a bit like fresh-released Microsoft softwares...
It is essential to verify that the recall was applied to that particular car (it can happen that a car is lost by MB, if it is sold and for one or another reason they lost it, especially if it was not serviced by MB).
Another thing, is the service-book; it is very important (also if you want to sell it after). Such expensive car have to be serviced by MB, and all that has to be written in the service-book. It also allow you to verify that the car has the good mileage and was not "miles-lifted"...
So, I would say, a 2004 or better end-2004 model with service-book and recalled, and you have an awesome car. It is, after the recall, extremely reliable, and a very pleasant car.
If you can have a FL it is even better, but it is not really an obligation. But reworked mid/end-2004 would be perfect.
The W211 is, after the recall, a real work of art. Class-leading reliability, a somptuous interior, fabulous comfort without beeing heavy, surprisingly fresh design, lots of elegance, large choice of excellent motors...

Never tried the 320 though, only 320cdi.
