My old Stilo 1.9JTD which is the great uncle of the 1.6 is a little bit noisy at idle but pretty smooth under way and as torquey than the last petrol engine I had, which was a 1.8 T/Spark @ 144bhp, vs 115bhp for the JTD.
The oil is 128p at the moment, which compares to 124p for the benzina. The beast sips it at a real 54-56 mpg compared to petrol @ 38mpg so a tankful costs me £60 compared to £50-ish.. but lasts twice as long. It seems cheaper to run, to me.
My beast takes 7 miles to warm up, so if you regularly drive 10 miles, it'll get warm enough to not bother the DPF or whatever fandango the 1.6 has fitted. Mine has an EGR valve that became clogged with soot and oil from an overfilled breather.. but once I hosed it all out, it's been fine.
I'm not sure the 500X has many vices... so it ought to be quite a low maintenance car.
The danger is that in future the Govt might increase fuel duty. To a certain extent they can't, otherwise all the hauliers will throw a fit.. so I reckon diesels will be safe from too much persecution for a while yet.... but more councils are declaring Low Emissions cash Generation Zones (LEGZ) so if you drive into any major city or town centre every day, you might find your diesel is as welcome as someone just off the plane from Wuhan.
If you only intend to keep it a few years, you'll probably be okay.
Ralf S.