Hi Sean -
My previous C-class was a W204 C250 CGI.
During my 5 years and 100,000km with it, I had some battery issues and replaced it once.
Roadside assistance was called when it wouldn't start, and they (NRMA) would NOT sell me a battery as the guy said it wreaked potential havoc with the car's systems. Beaurepaires and others will sell you something but there was the lingering adverse concern that you didn't know what "might" happen. NRMA guy said it was their policy with this car type not to supply a battery and I swallowed the medicine, ponied up to MB and paid $500+ for the battery and $160 for "software upgrades".
Ouch.
The second time it happened I went back to see them and walked out with a new W205 C200. It was easier!
I'm still worried that with the W205, I have no spare tyre, no can of squirt, and a MOE Continental run flat on each corner that is corner (and front/rear) specific. This madness extends to their soft SUVs ...
But I love the car. I just don't want a flat (MB insurance covers one new tyre and rim per year).