Bringing a boat from Norway to EU

[Aldis]

I am buying a boat in Norway and going to bring her to EU. The boat is in Lofoten and it will take time to sail her down south. Do you happen to have any experience with such procedure? I assume that once I buy her - the previous owner has to de-register her in Norway. I am going to register the boat under polish flag. When do I pay the customs and taxes like EU VAT? when entering the EU waters? Can i register and insure the boat while she is still in Norway so I may sail under polish flag? Thanks in advance!