I have 300 gal gas & 300 gallon diesel gravity flow barrels. As someone said, most bigger farmers are going to 500+ gal tanks with electric pumps, so you can get these gravity setups complete for $50-100. As someone else said, you can get off-road dyed diesel cheaper as it is minus the road tax.
Lots of electric pumps on gas tanks around here, but gas reacts much quicker to sparks, so you need the right pump & the right anti-static hose. I'll not offer more advise than that, as I don't know....
Another note, EPA regulations are getting stronger every year, while you might not need a containment structure right now, if starting from scratch you might as well install one. In my state a fuel spill of more than 10 gallons will sometimes cost you $12,000 in soil sampling & fines & all. Your insurance might well mandate a secondary containment under the tank(s) if the govt does not yet.
--->Paul