All OE tanks ARE plated to prevent corrosion. It's referred to as terneplate, a zinc coating.

You can build a tank out of mild steel, but it should be coated with a corrosion inhibitor. Fuller-O'brien used to make a product called Aviation Tank Sealer, it's a slushing compound. If applied to clean steel it will not peel off. I'm sure there are other brands, I've heard that Eastwood sells one, but I have no experience with it's dependability. You might check with marine supply stores too.

You can minimize the accumulation of condensation in a tank by keeping it full, but that's not always practical. Also avoid buying fuel when the tank truck is making it's drop. Otherwise you can take your chances with a bare steel tank and keep replacing filters as they plug from the crud that will form. It's especially fun to learn that your filter is nearly plugged when you pull out to pass a spud truck on a two lane highway and your rig falls on it's nose as you punch it.