Default X7510 in australia

where to buy, what to buy, which network to be on

i am thinking of buying x7510. not sure which version (US/ taiwan version),
Don't know which network to go on (optsu/ telstra).

I am going to use this as a laptop replacement and netbook. I was primarily with optus, was very excited that they are expanding their 3g 900Mhz hsdpa in regional areas but not sure. was thinking about changing to telstra for next G access to use outside of metro areas.

Also, where do you buy it in aussie?

Any input will be much appreciated