I suggest, as ilikecats said, to put something that you like down to avoid the late fee (hefty $150 or so). By putting something down (even though you're not 100% sure on that option) it will avoid the late fee and hassle. You can change it even after you obtain your ATAR.
96-97.
As regards to your second question, it depends on what you get in your externals in regards to your cohort.
Typically, if you do better than you did at school it will boost your mark up.
I believe an N award is given if you don't do the required hours or do not complete/do not give a serious attempt at an assessment. That being said, if you make a serious attempt in this exam (work through a couple of questions and re-read examples done in class) you will not be getting an N...