Having been there I concur it is a very interesting experience. Worth it to have a drink and watch the laser show.
The link posted in the comments above handles this more accurately. If your model iphone is a world model with a SIM card slot, with support for frequencies used in New Zealand, and your phone is unlocked by Sprint it will probably work. Sprint has had some changes to some phone to prevent unlocking but this may only apply to domestic carriers. If your service contract is up then they may unlock your phone if it is one that can be unlocked.
This is incredibly misleading. You hinted at the best answer. Inside the fuel filler flap it will read "unleaded gasoline only" or "diesel only". Usually with a minimum octane rating for gas and maybe a centane rating for diesel.
Since MXP is in the EU does that make any difference?