When I started my business I registered for GST from the start (though you are right, it is optional below a certain amount). You then charge your customers GST, but you can also claim back the GST on business expenses (like the stock you mention)
If I were you I probably wouldn't do anything about last year and would just start afresh with the new financial year. As pp mentioned, do your BAS quarterly and put aside tax with every incoming (I put away 1/3 but that is an overestimate so that I get a 'tax return' - forced savings lol)
Usually in the second year you are running the ATO will give you a PAYG amount that you pay each quarter based on the previous year.
I went to some of the free ATO workshops for small businesses when I started, some were boring and some were good. I do all my own tax now, easy peasy