If you don't know what you want, I would reccommend a double degree because it really does keep your options open whilst granting you useful qualifications. A good thing is that at least for now, you only need to sort out your Victorian preferences and then apply for ANU seperately.
Distance to where you live is a pretty big factor to consider, although 40mins isn't so bad. If I went to Melbourne Uni it would have been at least that far from my house. The only thing is that you won't be able to go home during the day, so you need to have that in mind when timetabling. Also remember that many lectures are recorded, so if, for instance, you only have lectures one day, you don't need to go in at all, really (although, always go to the first and last lectures). I'd also recomend public transport because then you can use your transit time for reading/studying, and it won't feel like time wasted. In that way, if you can get into Monash it would be a great option.
As for ANU: It's definitely more expensive to live away from home, however your regular expensives should be no different. Unless you do really well or are disadvantaged in some way, it's difficult to get a scholarship. However, it is quite easy to find casual or even part-time employment. Also, if you can get Youth Allowance, I find that most people who are paid by Centrelink live comfortably in terms of finances. There's also parents, if you can win them over. ANU also has loans at the Welfare office (I have no idea how good they are) and NAB and Commonwealth have some low interest student loans.