If your monthly running is just 500km and all of it is in the city, a diesel car is not the best choice. Diesel cars are best suited for those who cover a lot of distance on a regular basis, as in that way you will recover the initial extra outlay that you pay when buying a diesel. There is also the matter of the diesel particulate filter (DPF), which is a device that burns off trapped soot, but it does so only when it gets hot enough.
Typically, this is when the car is driven at least once a week on a drive of more than 70 to 80 kms and at higher speeds around 55 to 60kph. If this is the case with you, then it should be fine; if not, you will get a prompt to do a DPF regeneration cycle with would involve a drive as stated above or idling the car for about 10 minutes with the engine above 2000rpm. Since you already have the Scorpio N, we would recommend you keep it for a while and see if this is an issue or not. In any case, keeping the car for a few more years will not greatly impact the resale value.