Yep, it's very expensive, even when importing from somewhere else I'd still pay 3-4k€ due to the EU emission taxes...
Thanks for the tips, I'll pay attention to the stuff common for other cars as well, was mostly interested in something R50 or Nissan specific since I never owned one myself, so the valve screw thing will be very helpful. Looking at the topic it doesn't seem like something I could quickly check myself while inspecting the car, but I can only hope it's the original engine and that the odometer wasn't rolled back.