Find a fuel tank cleaning and restoration service, have them inspect the tanks before you buy.
Here is a good article on fuel tank issues written by someone who posts here.
https://stevedmarineconsulting.com/cleaning-diesel-tanks
Thanks, here's one on tank design as well. Valuable if you are going to have a tank fabricated.
http://stevedmarineconsulting.com/wp-content/uploads/2014/03/FuelTankDesign.pdf
And another on tank installation
https://stevedmarineconsulting.com/fuel-tank-installation/
Among other things tank tops should be sloped inboard to shed water that leaks from decks or vents. A well or sump built into the bottom is a very nice feature, along with inspection ports for each baffled chamber. Adding these features when a tank is built is not very costly. No hygroscopic materials should make contact with metallic tanks, and that includes non-hygroscopic materials such as Starboard that can trap water against a tank surface. Threaded tank fittings should use welding bosses, rather than pipe couplings.
I would strongly recommend
against a bladder for dealing with a failing tank, you'd have to cut a portion of the tank top out, and cut out the baffles if you are placing it inside the tank, and leave no sharp edges, and a very thorough cleaning of the tank interior to eliminate odors, a considerable job to say the least. Getting the plumbing properly oriented and looking right is very difficult. They are pretty much impossible to clean or drain water from. I could go on. As another member noted, it would also be a huge red flag at the time of sale.
Most custom fab'd tanks today are made from 5000 series aluminum (I prefer un-coated).
Fiberglass, as some have noted, is an excellent material for diesel fuel tanks, it's my tank material of choice. Ideally they should be VE resin or epoxy, and fire retardant, although many are made from PE resin with no issues. Nordhavns, Flemings, Hamptons, and a number of other vessels use FRP for fuel and in some cases every tank. Thus far I have encountered no problems with bio-diesel and FRP tanks in these vessels. Hatteras has used FRP for fuel tanks for over 30 years, plain old PE resin too. I've cleaned out some of these tanks, shoveling out the crud and hot water pressure washing the interior and the tanks look like new inside afterward, something that can't often be said of steel and aluminum tanks of that age.
Every gas filling station's tanks in the US are fiberglass, albeit a proprietary formula made by Xerxes (I called them to ask what it was but they wouldn't tell me), and some are used for 70% ethanol.