Hello,
While looking for the Ducato I came across 2 Iveco Dailies with one seat only. One of them had a cabin completely rebuilt by the converter (no B-pillar, so no fixing points for the passenger's seatbelt) - there was no possibility to revert this change in a costly reasonable manner (they would request a crashtest after the rebuild...). The other one had only the passenger seat + seatbelt removed and covered to make access to the cab from the right hand side much easier. So in theory it should be possible to easily revert to the factory state. However both vans were homologated for 1 person only. And that was the problem.
In such cases the homologation has more than one stage. The factory issues the primary homologation (with 2 or 3 seats in the case of those Dailies), then the converter modifies the van and rehomologates the vehicle. For 1 person in that case. It's a secondary homologation. Every substantial change requires next homologation by a certified company.
The local authorities told me that I'd need to buy a new seat + seatbelt at the dealership, make the conversion (install them) at an authorized workshop, and make a third homologation. Or at least some kind of basic check by an authorized automotive expert confirming the changes were done according to official rules. Plus of course all changes needed to be documented by invoices, so no own work, like installing the seat would be some kind of rocket science...
So, I bought a 3 seat Ducato
And then I bought a single passenger seat, as lowering the number of seat is much easier.
I've heard that there are companies that can do such modifications in Germany or Czechia (or perhaps Slovakia), but the vehicle need to be registered there, so again, not a cheap option.
I'd personally look for a van with a 7 seat homologation, might be wihout the seats in the 3rd row. But the paperwork should be correct for 7 persons. So you could install the 3rd raw seats before the first technical control in Hungary.
However the regulations in Hungary might be different, but only slightly, as you're still the part of the EU.