Hi, even though I'm not doing a commerce (international studies) degree, but I am currently on exchange and can answer your questions regarding exchange (as I know those that are doing an international studies degree who's also currently on exchange, and besides the $2000 and no need for credit transfers for int'l studies, everything else is about the same).
1)
University of New South Wales - UNSW International - UNSW Exchange Partners, UNSW International
USA or UK or any English speaking country is still ok to apply, even with the same language, they do have a different "culture" to Australia (say for example, halloween), the website lists all exchange partners you're able to apply to. Also as an international studies student you're able to apply to other universities not listed there (where non-int'l studies students cannot), but if that is approved, you will be liable for all tuition fees of the independent university you've applied for on top of the usual UNSW fee (whereas at a partner university you have to pay only UNSW your usual tuition fee like every other semester at UNSW).
2) Yes, popular countries and pretigeous schools can get very competitive upon application. Of course as an international studies student, you will be given piority in going on exchange, but there will be no guarantee that you will be given first or even second preference (there are 4). My friend doing int'l studies was given his fourth choice where as I was given my first (so there's no real favouring int'l studies in terms of university preferences), I believe the choice is made depending on your application quality (as you will be asked to write about yourself and how much you think you will benefit from the exchange to this particular school, I did a lot of research before my application about my first preference). As for your grades, they say you need a minimum WAM of 65 and you cannot fail more than 2 subjects up until your departure, but as far as I know of, your WAM is the last thing they consider for exchange. Even with a lower WAM, if you've applied for a less popular country, with less competition, the chances are you may actually be accepted (but don't quote me on that) - it's like demand and supply for the WAM. But for B. Commerce (int'l studies), I heard that you'd be automatically transferred to just straight B. Commerce if your WAM falls too much or you fails 2+ subjects? That I'm not really sure.
3) As for this, I've only just started my exchange. However in terms of cost really depends on where you're going to. For example living in Beijing would definately be cheaper than living in Tokyo. So sometimes the 1,800 AUD per month isn't really necessary. Also upon application you can submit a plan to them about how you might earn enough for your exchange within the year as you submit the application just about a year before your departure (keep in mind if you're on Youth Allowance, you should still be able to keep on getting it if not at a higher rate when you're going on exchange- as long as you tell them within 2 weeks of your departure; Even if you're not getting it right now, you might be eligible when you leave).
Phew that was a little long, hahaha I hope I didn't bore you with all that! And I hope it helped somehow!