There's no possible way you can know that with all honesty. Don't waste time looking forward to the future just yet, that won't change anything. Focus on there here and now. Focus on changing this outcome, not the perceived future outcome itself.
You need somewhere around ~70 to transfer depending on your ATAR and which uni you come from (anecdotally).
The short answer is that it isn't easy but it isn't hard either. You'll need to put in consistent work that might be slightly above your average student (a lot of people tend to really drop the ball and get lazy, so, average might not be as pro as you think). I've never heard of the Monash college thing, do you have more information on it? I honestly thought it was for people who haven't done VCE before.
As SBC said, your best bet is just to do commerce at another university. You'll actually be doing what you want (commerce) and right off the bat you'll get experience in what commerce is *really* like which will help you decide if its really for you (a lot of people change their mind and thats OK). It's more advantageous (annecdotally) for your transfer application because they've seen you can succeed in commerce. Also, Monash will credit you for the commerce units (usually) done at another uni. So, if you do accounting 101 at RMIT, you wont need to retake it at Monash, you can go straight into the next level. If you do an unrelated degree, you will have to start from the bottom.
TL;DR Try get into commerce somewhere, do well there then transfer over.