Our 2007 Vito has the same OM642 3.0 V6, with a DPF (not sure whether a 2008 C320 would have one?). These engines are not quite as complex as current diesels but there are still a lot of ancillary bits and pieces that can (and do) go wrong ... there's no way I'd be buying a 14 year old one for an elderly relative. From about 90K miles we had ...
Failed crank position sensor
Failed DPF pressure sensor
Failed DPF temperature sensor
Failed EGR valve
Failed turbo (caused by delaminating exhaust manifolds)
Failed MAF sensor
This is a one owner FMBSH vehicle. All of the above put it into limp mode, some of them multiple times until correctly diagnosed and fixed (by a main dealer). It's mostly used for long motorway runs, not short local trips (which as mentioned will cause DPF clogging).
It also gets through glowplugs at an alarming rate - these started failing at under 50k miles. Two more went recently, and the glowplug relay (which is not available from MB), and the special 225A 'mega fuse' fuse that supplies the glowplug circuit). Bill for this was over £1000 at a local independent (not main dealer).
Add in the recent hike in the price of diesel and the emissions concerns/uncertainties already mentioned and I would *definitely* be looking at a petrol engined car for mum. Our 'daily driver' is a 2.0 petrol C Class estate (W205).