Honestly I can't stand stainless tanks. It's just the wrong material for the job. Stainless by its nature is not really a good material to leave underwater, and even worse when half of it is submerged, and the other half isn't. It leads to selective chromium leaching, which causes electrolysis, rust, then destruction.
Of course some people have them, and the tanks have held up for them, but it's just not a good long term material.
For fresh water I would go with either plastic, composite fiberglass over plywood (food grade epoxy
), or titanium. Of the three plastic is probably the cheapest, unless you want to build it yourself, in which case the fiberglass would be. Titanium is likely the most expensive, but also the longest lasting, and has no issues with corrosion on the external side of the tank from poltice corrosion.
What I wouldn't do is spend the money on a custom stainless tank (which is likely very close to the cost of titanium) in 316L or any other super alloy. As the alloys get more exotic the corrosion resistance goes up, but so does the price. And frankly a titanium tank isn't that much more than 316 is, but has better corrosion resistance than even the best super alloys of stainless.
All that being said, for a fresh water tank I would also seriously consider bladders. They are cheap, easy to replace, and in the worst case where they puncture, it isn't the end of the world (this is my objection to bladders for holding tanks