An ABN will be fine, and is freely available by contacting the ATO.
Just be aware that by registering for an ABN you are indicating to the TAX department that you will be trading as a business, so you will also need to ensure you are keeping taxation records when you start trading.
You will also need to look at a business name if you are trading in any other name than your own, contacting your local licensing authority or consumer affairs in your state for full details.
If you just use an ABN to purchase, but do not do a tax return, it is only a matter of time before they come after you for the non payment of your GST quote, and at that point you will find yourself in a dirty big hole!