I've worked on a couple of W203's for some folks, just to help 'em out. It's possible that iCarSoft does enough for the W203 chassis to make it viable. Not sure, since I don't have an iCarSoft unit, but those who do have them seem to consider them a good value.
That is emphatically NOT the case with, say, the W220 chassis, of which we currently own four....or, in my experience, the W211 chassis, of which we own one. Those cars have enough bells and whistles that I think SDS really is needed to do everything that would need to be done for/to them. This is why I typically do recommend SDS for any MB owner. They're really not that expensive to get; US $600 is cheap compared to what I would have otherwise spent maintaining our fleet.