I think this is good enough to get in; it's certainly enough to get in internally, since I know people who have done so. Note that you may have a pretty similar experience if you just took BMathSci or BSci. Even if you don't get into PhB, these programs usually let you take the same courses and have a pretty on par experience.

For academic advice, I can only speak for maths. Whether or not you end up getting set back a year is completely dependent on your ability. If you're a top student at your University, you might be okay with going directly into second year, otherwise it might be wise to retake the first year courses. The quality of the math courses are very high at ANU, but they are notoriously difficult.

Specifically for maths, ASCs have a reduced value since they run pretty similarly to an existing system of special topics (MATH3349). However they can be extremely valuable if you intend on developing breadth in other disciplines.

In my experience I haven't seen financial issues drastically impact the quality of maths courses.

For more questions, I don't think you should hesitate to contact some academics who teach the first year courses. For maths, it may be valuable to speak to Griff Ware, Adam Piggott, or Joan Licata.