Also check ebay. I've got my 2ft tank off there for $15, and my 6ft was $270 (compared to $600 if I'd got it brand new!). A good rule is to go as big as you can fit in your house (or think you can fit... Yes, this is from experience
), even if it means saving a week or two more to get the better setup (though at the moment there are HEAPS of tanks up for grabs at quite low prices, so it shouldn't be too much of a wait!)
The specific sand you're looking for at Bunnings is Play sand. It's about $9 a 20kg bag, and has a good sized grain.
Also, if you want to put in live plants, they'll need time to grow, so have them in whilst you're cycling. Consider getting a few guppies to help the cycling process- your axie will have great fun when they arrive home chasing down any survivors