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ECU initiates a purge cycle to burn the deposits off the DPF at regular intervals. I'm not sure how often, maybe hundreds of km? You won't know it's happened if you're cruising around 100 km/h, but you can hear a change in engine note for a few minutes if it happens at lower speeds.
So if the car drives OK, I wouldn't worry about the DPF. The MB economy you've mentioned is pretty good for short commutes.
It's worth noting that the Ford oil spec. is NOT for a low SAPS oil, which should be used in DPF equipped engines. I've used a Total oil since mine was out of warranty. So a non-Ford service history could well be better, as they've probably used the "correct" oil. There are a few threads on this here.
I own a late model MB (love it, will be 5 years old next month), and am happy with the 6-speed auto. I use it manual mode sometimes, works well with the diesel. I've driven MC Mondeos and definitely prefer the MB auto around town. But the Powershift would be good for open road driving.
Personally, I'd avoid an early MC because of possible Powershift problems, although the Ford box isn't the same disaster area as some of the VW DSG efforts.
When you "kickdown" a DSG box, it will sometimes downshift more than one gear. That can feel quite harsh, especially with a high torque diesel. I would definitely drive both models if you can, the MB and MC do drive differently, plus the MC is a little more economical.
Good luck, I'm sure you'll enjoy the Mondeo, especially if you carry passengers and/or a load. Far more room than most SUVs - there is no comparison with something like a Santa Fe.