I was thinking also galvanized steel for pillars and plates. Have the sellers, have the prices. It is cheap. All steel material is about $400-$500.
I was playing a bit with SolidWorks to make a plan with complete steel thickness for pillars and plates (and statical analysis, bending under the weight, maximal bending of the shelves) ... but give up.
The hard part is to make it really water resistant (without welding everything together).
On the top easy, just a bottlecap going down a little bit on all 4 sides.
The side-rear panels can bend on the ends and overlap. No big deal.
But the door ... the frame has to be this beak nose ... and with a double door that sliding into that ... too much for me
True. I checked up some and I was in a lot server rooms before. They are deep and not wide.
Also full with holes (and even used ones are expensive)
Gun cabinets, safes, filing cabinets. Not joking already checking used items
But at the end if I am not too lazy (and buy the small battery and inverter cabinets) then I can weld the cabinet together myself.
... but I am too lazy and too busy for that.