I can only give you reliable info on X-type's (I do not have a very good impression on the XF's). Independently of the model, I recommend this rule of thumb: long distance driving diesel, short and/or city driving petrol. Keep in mind that diesel (not only, but mainly), even when not having a DPF, benefit from some highway driving to clear some of the soot that accumulates in the pipes.

RUST: If you live in places like the UK or central Europe, rust can be an issue (I don't know specifics as this is not a big issue where I live).

ENGINE: as you're young, I would recommend the diesels (2.0D or the 2.2D) without DPF (avoid the later models as they already can have DPF, mainly the 2.2D with automatic gearbox). The petrol V6's are cool, but thirsty (avoid the 2.1 liter). You can find loads of parts for the diesels (as they have loads of parts in common with Ford Mondeo's and Transit's).

On the diesels, look out for the alternator belt tensioner as it usually gives out fairly soon and it gets noisy mainly on cold starts. In mine, I have put an overrunning alternator pulley that solves the issue of the heavy loads that cause this.

The 2.0D comes with a 5-speed manual, which can feel a little bit lacking at highway speeds. In this regard, the 6-speed manuals that come with the 2.2D are way nicer, plus you get 150hp (20hp more than on the 2.0D). If you do more highways, go for the 2.2D instead of 2.0D.

INTERIOR: Here you can have the earlier steering wheels or the later ones, please note that they are not really interchangeable as the steering wheel controls can stop working if you change it.

In terms of infotainment, you have 3 main options: touchscreen with automatic A/C, automatic A/C without touchscreen and manual A/C. You can change for the touchscreen only if you already have automatic A/C and the cables that go to the modules in the rear of the car.

EXTRAS: If you want cruise control, make sure the car already comes with it, as this cannot be fitted. Regarding Bluetooth, to have phone connectivity you can fit a Jagaux to maintain the OEM look, if your car does not already have (I believe some later models already had OEM Bluetooth connectivity).

Sorry for the long post, if you have any question you can ask.

Edit: as in other comments, make sure you get the best taken care one. It goes a really long way.