You are probably aware of this but I'll mention it anyways: there is a very good chance (in fact, I'd wager a guess and pretty damn close to 100%) that the Australian PSPs will be flashed with a post-1.50 firmware (most likely 1.53 or whatever revision they can come up with). So, buh-bye homebrew.
The whole warranty situation is indeed a concern. I imported my PSP and I was worried about the very limited warranty. You can find good deals with 6-months-warranties online (mostly from HK-based dealers). But there's a good chance that you'll pay extra for S&H and more importantly customs. Having a local dealer has the obvious advantage of being able to check stuff out (ie. checking for dead pxiels, verifying that it's really 1.50 vs. just checking the box, that sort of thing).
In the end, you'll have to decide for yourself. But as leering said, the differences between the Japanese PSP and the European/Australian version will most likely be miniscule. That may change if the EU/AU PSPs come pre-flashed with a new firmware but that's not really a problem since you always have the option of upgrading (at the cost of losing the ability to run unsigned code).