You should look into a commercial hollow metal door, 14 ga, top hat reinforced, with welded seams, in a welded 14 ga frame. It's commercial, and Republic, Steelcraft etc dealers can supply. When installed use the anchors, and grout to fill the jambs completely, support the top with blocking to keep it from settling with concrete in the header.
Simplex lock to operate a mortise works, not cylindrical with upper and lower deadbolts if desired. Three interlocking security hinges minimum 4 1/2" with bearings and a pinned hinge pin to prevent extraction - even if its inswing.
We had a youth detention center go thru an 18 ga, then a 16ga, a 14 year old client who weighed 240 pounds literally pounded the first out of the wall, the second he ripped the skin off the corner and folded it up. The 14 ga, installed correctly, could not be defeated after 24 hours of experienced effort, and held him for the duration of his care. There were no further efforts to defeat it after that.
Of course a torch, hydraulic cutter or powered carbide saw would still cut their way thru. The idea is to buy time, and the most resistant to human powered attack with few tools get the better ratings. An experienced commercial door supplier who bids government jobs should have no problem getting one. No effing way a lumberyard or box store will do it unless they buy it from him first then double the price. Residential cannot buy commercial direct, save money and just find him yourself. Hardware too - in the day we sold commercial hinges for $5 a pop, they were $20 at the lumberyard.