You should be talking to your builder now about how you will run the vent to the outside wall "invisibly". Had an issue with this myself when I retro-fitted an island hood to the kitchen, ended up having to run the vent along the underside of the ceiling.
Perhaps builder could also install a 4-6" pipe through the blockwork in the wall to run your vent through.
Island hoods are pretty difficult to mount yourself (very heavy, at height etc.) so if the builder / kitchen fitter will take it on, I suggest you let them do it.