Ezz,
If you sign up for an approved 150hr integrated CPL course, you don't pay GST. Not sure about the tax ramafications beyond CPL, torque seems to be on to it though.
You do not need an instrument rating to get an instructor rating, the prerequisite is CPL + NVFR as a minimum. I chose to do an instrument rating over NVFR as this allows you to exercise the priveliges of a NVFR as long as the rating is current.
You really just need to decide which path you want to go down though - charter or instructing. If you decide to go down the instructor route though, make sure it is something you actually want to do and not something you are forcing yourself to do. Nothing worse than an instructor who doesn't give a damn