What are you looking to do? If you're just looking to coat a tank with some rust inside but that is structurally sound, buy the small tank sealer on it's own. You can use other products (search this site to get an exhaustive list) to remove the rust, then just use the sealer. If you've got some holes I'd still go with the approach above but use some jb weld or other similar repair (again, search this site for other options) then seal and paint.
The POR-15 sealer is excellent stuff. You won't go wrong if you want to order their entire kit just for peace of mind.
Good luck.