there is an ATM at the airport in Nadi just as you come out of customs. many folks just line up there and get some cash out. I would always take a bit of cash for in hand (just for emergency stuff...ie if you need a taxi for whatever reason or want to stop off for a snack somewhere at the airport if you need to wait for the transfer...my 'safe' amount is $100). i am assuming though that you will be going straight to a hotel? they usually have ATMs (but best to check on their website). Many hotels there do have some sort of internet but be warned as it can be realllly slow and the humidity does tend to do weird things with the keys lol. In all honesty i dont think you will need to use it that much. we use our normal debit or credit card for getting cash out and for purchases etc. if you are mainly staying at the hotel they can bill everything pretty much to your room and you just pay at the end with your card.

as for luggage, seriously Fiji customs are pretty easy i would lock my bag just like Onyx said for the opportunistic thief but also JIC the zip does become caught and might unzip (has happened to me in the past!).

as for phones there is a store right outside customs where you get the vodaphone pre paid sim. i would go that option.

HTH! have a great time!!!!