Standing at the edge of Geirangerfjord with the sheer green cliffs dropping into deep blue water is worth every bit of the hype. Summer is the right time to go, when the days are long and the ferry routes are fully open. Booking four months ahead gets you the best shot at decent places to stay and reasonable flights before the summer rush hits.
A trip like this usually runs about $3800 for a standard US traveler. Spreading that out over time makes it easier to swallow, which is where financing comes in if you want to stretch out the payments.
That 0% window is a deadline, not free money. If you still have a balance left when the 18 months are up, standard interest kicks in on what is left over, so make sure you pay it off before time runs out.