The question is whether I can safely and efficiently use just a citric acid I buy in any shop for decalcification? Or is there something else readily available I could use?
The manufacturers will tell you:
A similar reason goes for vinegar. And Citric acid (from what I'm told) is less problematic than vinegar.
The real expensive stuff like Durgol isn't actually citric acid and some other formula (unlike citric acid it tastes awful).
Here is their description (pdf)
While your machine's specific kit might be dosed for perfection, you're likely ok with any descaler for the size of the machine. A competition grade professional machine on the other hand, I'd recommend vicious service. If you only see what's under the shower screen of some of the machines out there in the wild.
A key point is that they claim their product is faster. So if you have time, and are willing to rinse a quite a few more times, I wouldn't worry too much about it.
The Cafiza powder from Urnex is really good at cleaning coffee grime, but does not de-scale/de-calcify. I would recommend it for cleaning coffee urns and the coffee stained parts of your machine.
Finally, you might wish to use softened water. It'll make the coffee taste better and reduce the build up.