Epilogue: It would have been smart to take serious rust precautions for the sunroof. After 6 years, the vinyl roof is now off again (and will not be replaced), the sunroof is extremely rusty due to poor workmanship (and was the first thing to break anyway due to the driver cable tearing off). In this respect, of course, it fits in with the rest of the vehicle (silicone instead of a proper seal for the differential, one of the differential bolts was also torn off), defective drive for the headlight flaps, non-existent valve stem seals, warped timing chain cover without a proper seal, wrong installed carburetor, rust hole in the rear right wheel tip, ... you name it).
I'm still angry that I bought this rubbish for so much money. In addition to the 15,000 euros plus freight and customs, I spent several thousand euros on repairs and spare parts. That's really not what I'd call a "good job".