AlfredB I wouldn’t worry the boiler will be AISI 316L stainless, so perfectly safe to descale, no descaler will attack the seals. The issue around descaling is simply getting the descaler out! The best way will be via a big syringe/tube and removing a fitting on top of the boiler. Don’t remove the heating element if some wag suggests it.
It doesn’t sound like you will need to descale your Puristika and the best advice is to make sure you don’t have to descale..,.ever. I use an Osmio Zero to make RO water, and a touch of bicarb to bring the water TDS up (sometimes). This means I never have to worry about descaling.
If you do ever have to descale, remember to lubricate the group inside immediately after…which is something you should be familiar with as you have to do it every time you flush with a cleaner like Puly cafe, evey few months. Oh and of course you are removing the shower screen at least weekly and cleaning it, and behind it?