How important is Italian “00” flour?

Not especially, particularly for the first-time Neapolitan pizza maker. Funny thing about the pizza making arts, most people who are just beginning to practice assume that the magic is all in the flour (or some other secret ingredient), when the trick is really in the technique (the handling of the dough, the heat of the oven, the preparation of ingredients, etc). Certainly you can start out with expensive Italian flour if you wish, but you likely won’t notice it until you’ve tried your hand at a few pies.