We have used the maxon modules in the past, that device is around $400AU and whilst it does WLAN, it doesn't do GPS. The Sierra Wireless card is Telstra NextG approved.
You will require any SIM card with a data plan, or prepaid with data allowance. You can get NextG compatible pre-paid cards with different allowances on them. If you are also going to allow your CarPC to share the data connection, you might want to get at least a few Mb, and disable automatic windows updates.