>>8675097I recommend between 16 and 12 gauge mild steel, either cold rolled or hot rolled for all the body armor. This is because the Mandalorian body armor is mostly flat shapes; there aren't any deep domes or complex curvatures like that would be found on medieval armor. Flat shapes are very easy to form so you can afford to go with heavier stuff.
Going off the specific design shown in your picture:
Shoulder pads - 12 gauge because they are very distinctive and the edges of this armor piece will be visible the most often. If it looks thick and beefy, it implies the rest of your armor is thick and beefy. Won't require much works despite the thickness because it's only lightly curved in one direction.
Neck and chest - 14 gauge because even though they aren't as curved as the shoulder pads are, they are curved in more than one direction. Also, you want a nice transition between the 12 gauge shoulder pads and the 16 gauge arms, groin, legs.
Arms, groin, legs - 16 gauge because these armor pieces won't be noticed as much and also because their tight curves mean that if they were any thicker, they would require a good workout.
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>>8675326) is right about the weight of the steel plates. You will not want to attach them directly to your base fabric layer. Instead, you could rivet the steel plates to a sturdy vest made out of canvas, and then wear the vest over the base fabric layer. The shoulder plates each have a strap connecting to the vest along the top of the shoulder, then another strap that encircles the arm. The groin plate connects to the bottom of the vest. Then there needs to be straps running from the groin plate to the top of each thigh plate to hold them up. Similar to the shoulder pads, each thigh plate also has a strap that encircles the thigh.
