The required textbooks will be clear to you after your first lectures. The lecturers will either tell you, and/or state it in the course outline that will be distributed to you.
If you want to purchase the textbooks early, bear in mind that the required textbooks may change as the lecturers sees fit.
Whether the prescribed textbook is necessary is up to you. Different lecturers rely on the prescribed textbook to different extents. Most of the time however, tutorial questions are set from the textbook. I tend to rely on the textbook readings rather than lecture notes.
There is no haste to purchase textbooks: assessments do not come until some weeks into the semester. However, if you like to keep up-to-date with the unit then you might want the textbook by the first tutorial (usually second week).
You might also want to consider alternatives to first-hand purchasing: second-hand, sharing with associates, library copies, photocopying the required sections of the textbook...