The VCAA papers haven't changed "too" much from 2005 to 2008 (or whatever the period for the directly previous study design was, earlier than that one and you get pretty significant changes in terms of the structure of the course), you should be able to find you're able to do most of the questions.
I think there should be 15-20 exams available for the new study design, so doing the old wouldn't be really worth it.
I'd disagree, it's probably better to prioritise VCAA papers over the company papers, especially seeing that company papers are a bit of a joke every now and then.
If you don't want to actually do the exams, then you'd be doing yourself a disservice if you don't at least look at the VCAA questions and read what the assessor reports from 2002 onwards. There's a fair bit of valuable input in there that you'll come across.
Doing the 2005 VCAA exams onwards is something I'd strongly recommend, and picking out the relevant hard looking questions from earlier than that if you want. You'll want to be keeping an eye out for no longer relevant content though, you should be able to recognise that easily enough.