Umm why do you need another 2 units. Most schools don't cater for 14 units unless you're doing both extensions or a tafe course.
You've chosen very very difficult subjects. For most people chemistry, physics and economics are the hardest subjects. Personally I would say physics and chem are harder then economics but that's because I'm not particularly science minded. My sister and boyfriend both did business studies and said it was excruciatingly boring but not particularly difficult.
I'll use the University of Sydney for a benchmark here. Generally because it's the most difficult university to get into in New South Wales, if not in Australia. For B Science (nutrition) you need a UAI of 93.8. They assume you have knowledge of Mathematics of Maths Extension (depends on what subjects you choose). They also assume you know chemistry and physics of bio. Again it depends on the subjects you choose. University today is very open ended... they no longer have prerequisites and have bridging courses over the summer... so if you don't have the knowledge you can gain it so you're not behind. So if you decide you do want to do nutrition and want to major in chem/biology not physics then you can do the bridging course over the summer before university.
If you want to do a B Applied Science (exercise and sport science) they them assume you know Chemistry and Mathematics with the Recommended Studies of Physics, Bio, Maths ext 1 and PDHPE. The UAI for that is 87.35 at USyd. But remember, most other uni's are lower than Sydney.
I did none of those subjects bar English Advanced, Mathematics and I did Preliminary Chemistry. I despised Chemistry and Mathematics. I dropped chem as soon as I could, I thought it was terrible. Mathematics dragged my UAI down by about 5 marks. But again, I'm not science/maths minded. It's not my thing. So I can't exactly be used for a benchmark on the subjects.
Is engineering a good subject? At my school it had a terrible teacher. My boyfriend and his best friend did it. His best friend was also the dux and his least favourite and worst subject was engineering. I think a lot of people think engineering is a bit of a bludge but in reality it is actually fairly hard.
For scaling... don't even think about it. I chose to stick with Mathematics because I was obsessed with general scaling badly. In reality I would have been much better going really well in general then going a bit below average in Mathematics. My worst scaling subject was Ancient History. It was my second best mark. My best scaling subject was Mathematics... it was my worst by a LONG way. Scaling is irrelevant. This site however does have some great, if not outdated, info on scaling http://www.boredofstudies.org/scaling.php
But again, I urge you not to worry about scaling. The dux the year before mine did histories, society and culture, legal studies... all subjects which traditionally aren't "scaling'. She got a uai of 98.
Your UAI is definitely achievable. I wasn't particularly studious in year 12. A fairly run of the mill student really and I got a UAI of 87.8. Don't epic fail at Maths like I did and you'll be fine ;)