Yes, of course you can. You can use the files for pretty much anything you like. But you cannot use them for uploading to other stock sites or selling the image on as is (or with minor modifications) as if it was your own.
No. You pay once you can use the image and you do not ever need to pay anything else again even if you have millions of views of your finished product that includes the image.
Yes. You can modify the item to suit your product/use.
Yes.
Yes.