An Australian PSP charger will work just fine with the Sony Reader. The important thing is not the country, or input voltage, or frequency, it's the output voltage. All PSP chargers should put out ~5 volts DC.
Regarding memory cards: The readers internal memory will hold a lot of books. You should not need an expansion card unless you intend to load your entire library. The downside to having a lot of books on the reader reveals one of Sony's quirks. They have the reader perform an automatic scan of the readers memory, internal and cards, every time the device is disconnected from the USB. The time required for this scan is dependent on the number of books you have loaded. It can take from less than a minute for a few books, say less than 100, to hours if you have the memory full. Just because you can load thousands of books into the expanded memory does not mean is is a practical thing to do. I limit myself to about 100 books at a time. Since I'm one of those people that only reads one book at a time, delete it, and start on the next book, 100 is still more than I can justify carrying around. I use Calibre to maintain my source files on my PC.