Unfortunately I can't vouch for amount used in a van, but in a Cherokee I use to own it took pretty much a gallon to do the full floor, with the rear cargo area getting 3 coats and the rest getting 1. I would have liked to put it on thicker if I had more.
I used Herculiner, and I think its really important to prep really well for it, maybe even use a primer. I followed the instructions and roughed up the floor (most of which was fresh sheetmetal after replacing due to severe rust) and cleaned it well with acetone, and I felt like the herculiner would tear and peel too easily if you put something heavy and hard on it. It did not adhere as well as I hoped it would, and was not even remotely close to the quality of LineX or a professionally done coating. I also used the cheap aerosol stuff years ago as well on a different project and that stuff is worthless.
This was all 5 or so years ago, its possible they have improved. Perhaps other people have had better luck. If you are just going to be walking on the floor it will probably be fine.