If you worry about DPF, a diesel car is not for you.
Mazda diesels are very sensitive and require close attention to the injectors even if the car runs "fine".
You should not rely on service history and service intervals and hope for the best.
You need to be proactive with these things.
If the injectors are fine and you use it for proper driving, you will be fine.
For the mods/electrics/etc, the 6 comes with most of the extras already and what you can add as an upgrade from the next spec up, it will cost you a fortune to fit it and make it worthy giving a few extra minutes locating the right spec from the very start.
The Saab is ancient platform and the 6 will not work out like that. In the old cars from PSA, VW, GM, etc, you could buy a cheap version with no cruise for example and then play around and fit. Nowadays the cars are fully loaded in that sense. You can buy a basic 6 with navigation but you can find an upper navigation spec for the same money. So it does not make it worthy not buying what extras you want.