Spear Golem: Taking a single soul and implanting them in a new body would cost the amount of the new body, assuming this isn't abused. Making it on a large-scale would be a base-afterlife of reincarnation (In which the souls would forget everything, essentially being new souls for most purposes).
.001*(PT Metal+1)(*1.2)(CP Spine)(*1.4)(PT Weakpoint -2)(*.7)=1.17E for 1000
Assuming fairly week metal and spine. Magic would have to come from a separate magic system, with an extra (CP Magic Affinity+X) to give them a greater ability with it. A super-high (CP Magic-like-ability+X) can act as a magic system for most cases, but you would have to describe the exact ability of the magic system. The above is only if I am understanding everything right. As always, you can add more +s to make the abilities stronger, or minuses to make the weak point more of a vulnerability.
Well, the original metal would either be iron or steel, taken from the environment (just melt some cars).
The magic mass would be damageable, but unless the eye is taken out, they'll eventually regenerate.
If one were to cut off it's arm, the severed mass would dissolve, dropping any armor on it, and metal within it. Without the magic to liquefy it, the metal turns solid.
It would generally take a week to regrow a limb, the stronger the magic mass, the faster the regeneration.
The magic ability is used to liquidize metal through magic, and manipulating said metal... whenever inside the soul mass.
Also, the soul mass is very squishy. Even though it could hold tons of armor, you could sever an arm with a wood sword.
The eye on the other hand, requires a hard hit with a hammer to break in one attack. But it can't regenerate, at all. So you could just crack it until it breaks.
And any metal that enters their soul mass wont damage them (unless touching the eye), even if you shoot it at them.
This also allows them to submerge parts of their armor into their body, and make it all fancy and stuff.
They can change the form of their soul mass, but only if they focus enough.
The soul itself retains all memory, as the intended purpose is to protect those dear to them, even after death.
Though they do so in a body made purely for killing.
Koroshiya will likely only do it to the souls of the fallen in a certain area, making a group of soldiers in a very short time.