Mathematics Extension 1 is largely sufficient for a Bachelor of Business/Commerce with a finance major. Mathematics Advanced is also considered to be sufficient since Finance requires little knowledge of Mathematics Advanced content.
It is good to be taking Business Studies and thinking about studying a Bachelor of Business/Commerce as knowlege gained and concepts learned in this subject (and to a lesser extent, Economics) will likely be revisited, particularly in your first year and (possibly) in your second year. This likely depends on the major.
You can definitely achieve high marks in a Bachelor of Business/Commerce, irrespective of the major, although one of the main differences is that people who like maths may find that majors involving maths may be easier to achieve favourable results in, as opposed to those who don’t like maths, who may consequently find the Finance major difficult to achieve good results in.
While a 6.5+ GPA is definitely achievable, keep in mind that this is a rather high GPA, meaning that it may be difficult to achieve. Further, your academic position (i.e. results + GPA/WAM) don’t depend on your lecturers to the extent that they depend on the quality of your own work.