I'm of the same opinion as swarley.
Engineering scholarship at Monash plus you live close by, plus doing engineering, don't see a reason as to why you want to go to Melbourne. Hopefully it's not for trivial reasons such as friends there and the ranking of Melb uni etc...
As for the double degree, I've done science before and like you had liked Maths, physics and chem before going into uni. Not saying that you won't like it, but subjects in uni are a lot different from VCE. For engineering, I like to refer it as something similar to applied Maths/ physics that spins your head around.
For commerce, some employers prefer it while some don't. Do it if you have an interest in it but don't force it if you don't. (Ask yourself questions like do you check the stock market regularly, interested in managing projects, etc etc.)
As for biomed I'm not too sure about it.