I found your thread because I've fallen in love with the C303 and would like to buy one and do a camper conversion.
I see in the end you switched to a C304 -- I don't have a 7.5t license so I'll stay with the C303.
I also see you moved from Switzerland back to the UK. I'm still in Switzerland -- did you originally buy the vehicle and import it into Switzerland? Can you share any tips and tricks about importation and registration of the vehicle when you originally brought it to Switzerland? Did you need special permission to make modifications?