There is very little left over. I found when doing themthat I had less scrap than a traditional top to have.
I yet yet to do one it they also do an octagon one.
The fabric when choosing you want not a lot of background, a fair to large print. And depends what you like to the amount of colors. The more colors I find the more difficult it is to lay out the blocks.
With the repeats and size of project differing it is had to say a definitive on how much fabric. Bare minimum is to be able to find 6 repeats on the fabric. So if you are making a wall hanging or table topper you need very little as it is repeated width wise. Then depending on the sizeof project the more sets of repeats that you need. For a full size quilt top I would go minimum 18 repeats along the salvage. The size of repeat varies from print to print. So if it is 12" then times that by the 18 ( plus a couple for straightening) (6 yards) Then if you would like a border out of matching fabric add that as well. (yard/yard and a half)