From an Aussie (albeit one who has no experience with pre-pay), I'd say just buy a local pre-pay SIM card once you're here. Outgoing costs aren't cheap but pre-pay is always like that.

You can probably purchase a $30 card for $20 or something to get you started.

BTW: We use 900MHz generally. I think there are some 1800MHz sites around but I've never seen that advertised via the operator name e.g. in Singapore it will come up with something like M1-1800 when using M1's 1800MHz service.