Buying games on steam and good mac games on steam.

At the moment i only have free games on steam and i was was wondering about payment for games. The games are displayed are in USD but the money in my bank is in NZD. If i bought a game would i have to pay an extra charge because of the currencies and is their an option to change the USD on steam to be displayed as NZD. Also are there any good and cheap games that i should consider buying (Mac).