Just a note, it will only need to be engraved with an importer's mark if it goes through them (e.g. Borderview). If you go through the process yourself you can avoid the engraving, but it's a complicated process and if you move before it's approved the whole process is scrapped irrevocably. Other than that, ask Borderview - their cost is relatively reasonable and they are willing to place the mark in an unobtrusive location.
I still wonder if you can still use the 6NIA if you are not moving there _permanently_ - just keep renewing it every year?