Use heating oil tank as diesel tank?

Also read the TRGS 510. Assume that you will not get a building permit (which you need).

Setting aside the meaningfulness, why should different regulations apply here than for heating oil? It's the same thing, only that the diesel is not dyed.

Ask the farmer how it works. They also have their own gas stations for [DEN fuhrpark].

Oh yes, I just remembered that where you pump, you need a WU concrete base. So under your car / at the pumping station, you still need to do a lot of earthworks.

What savings do you expect if you still have to acquire the pumping station (how much does it cost? T€ 1 upwards for private or rather even more) and the "station area" (several thousand €) and partly also have to keep it waiting?

Well, with a 20ct difference, with 6000 liters you're basically at about break even. Of course, it's totally worth it. So if the gas station cost a bit over a thousand...

Chemically certainly no problem but the fire load

Diesel does not burn. Nor does it explode. You can use it to extinguish a burning match.

It's the same, only that the diesel is not colored

Diesel and light heating oil differ only in taxation and coloring. Using the tank is technically no problem or safety risk.