I'm 3/4 without 1/2. You'll be fine. You will need background knowledge of the atmosphere in relation to the greenhouse effect(s).
45+? It's hard to tell. There is less than 1000 kids in the state that do it. In 2015, there were less than 700. This means it's harder to get such a high score, because everyone will be getting high marks throughout the year.
As for biology or enviro, it really depends what you enjoy more. Biology is really content heavy but it's so much more interesting (imo). Enviro isn't as heavy in the content, unit 3 is good, but some (most) of the unit 4 content is super bland!
I've found that while my teacher cares about his class doing well, we all find it very hard to keep motivated for the exam, so the teacher is also a big factor.
If you're going to fast track one of the two, I'd choose biology. It's much easier to have environmental science in year 12 rather than biology, if you choose to do so.
Good luck with your studies