Here are my thoughts, and yes, I am also accused of over-thinking or having too much time on my hands.
Site the tank to suit you. Here in the NE some homes have two 375-gallon home heating oil (diesel) tanks in the basement; I have one 275-gallon tank. I did get a permit from the local FD (at the time it was issued w/o expiration date). If you have a friend in the FD, ask him, he has probably seen a lot. If no such friend, the fire marshall, building inspector, firefighter will give a pass-the-buck standard answer. Shade will reduce the heating and cooling cycles that increase condensation.
Raise the tank on blocks or some kind of frame. This will give you room for the drain plumbing, and room to view weeping or other signs of impending failure. Mitigating a soil contamination incident may be as problematic as a fire.
Tilt the tank slightly. This allows Bottoms, Sediment, and Water (BSW) to collect at the low end where you can tap it Sediment may collect all over the bottom, but a slight (1") pitch will facilitate flushing if you have to do that. Make sure the pump suction is at least 2" above the bottom.
Put a nipple and cap below the ball valve on the drain. All valves leak, and a ball valve lever can be easily hit, cracking the valve. Periodically open the drain into a vessel such as a one-gallon glass jar. Let the sample settle and see how much BSW you have. Decant or siphon good oil off the top of the sample and put it back in the tank. How long you wait between samples will depend on what you find {and how much time you are distracted by studying oil futures to get the lowest price each 3 year cycle). Given the infrequency of refilling, the tank will act as a settling tank extending the life of your filters. In perusing the web sites of the snake-oil salesmen (Power Serve, Lucas, Bell) I note they "disperse" water and contaminants i.e. hold them in suspension (until they are filtered?).
The oil companies/distributors have added their own snake oil prior to filling your tank. I would treat only as problems (such as algae) arise. Secondly, I would treat the diesel in the fuel tanks of the vehicles. As suggested, I would buy only summer diesel. (PS literature warns against using their cold additive year-round.) When temps fall, treat the vehicle tank plus a couple of 5-gallon containers. If the fuel in the storage tank gels use the treated fuel in those containers while you heat the tank. There are electric battery blankets, water line tracers, etc. even an incandescent bulb with some space blanket shielding may raise the temperature enough to pump.
