Looks good!
Updated.
Challenge Rating: x
Challenge Rating 15? It's 6HD and a size larger than a CR13 Iron Golem, and has a DR20!
Advancement: 25-x HD (Huge); x-x HD (Gargantuan)
An Iron Golem goes 100-150% HD, 150-300% HD. If the Mithral's the same it'll be:
Advancement: 25-36 HD (Huge); 37-72 HD (Gargantuan)
A lesser mithral guardian stands 18 feet tall and weighs x pounds. (Note: Mithral weighs half as much as steel)
Going by the weights an Iron Golem is probably roughly 50% hollow, so if the Mithral Golem is solid they'll weight the same.
Still, I'd prefer to think their construction is similar, in which case an 18 foot Mithral Golem should weigh ~8500 pounds.
Construction suggestions?
Total price around 200,000-300,000 gp?
An Iron Golem requires 5,000 pounds of pure iron plus tinctures with a material cost of 10,000 gp. Mithral costs an extra 500 gp/pound, which would work out at 2,500,000 gp for that amount of metal, or 4,250,000 for the previously estimated 8,500 pound golem.
So, how about manufacture requires 8000 pounds of pure silver and 100 pounds of mithral, plus assorted other metals and rare tinctures, with a total cost of 100,000 gp , and the construction cost is another 100,000 gp, for a total cost of 300,000? [That works out 40,000 silver + 50,000 mithral + 10,000 miscellaneous]
It'll also need an impressive Craft (armorsmithing) or Craft (weaponsmithing) check - maybe DC23?
For spell prerequisites I'm fancying
sunburst and
permanency together with the usual suspects of
geas/quest,
animate object and
wish or
limited wish.