They're one heck of an intelligent (if single-minded) race of constructs to me. They have an even narrower mandate than inevitables.
I vote for "breath weapon" for the cone of cold, and agree to dropping/revising the "sleep for a thousand years after answering".
The infinite height should probably be changed to just make them seem infinitily tall, but stick with a standard space/reach.
Should Str be "-" or 10, since they really can't move or physically manipulate objects. I'm leaning toward 10, on the off chance someone uses an attack against them that requires an opposed Str check (good luck with that, though).
I was thinking we could give them some kind of spatial distortion ability so that no matter how much you climb/fly up them you never reach the top.
I'd give them a Str score. They keep treasure inside themselves, so they presumably can pull it into their bodies somhow. Maybe they've got some kind of marble tongue?
They can answer questions, so do they have voices or are they telepathic?
Let's try a modified version of the epic spell reflection. So all named spells are also useless against it?
6" cone of cold translate to how many feet?
Should they have an extradimensional space in their insides that functions like a bag of holding or are they voluminous enough to make it superfluous? (There's no diameter mentioned, 10 feet across perhaps?) I agree with the Str 10 score.
The spire in the center of the Outlands is infinitely high, right, so if we can find some flavor text for that and Sigil (Planar Handbook?) we can use that.
Let's hammer out the basics before proceeding further with the special abilities.
Colossal Construct (Extraplanar)?
Str 10, Dex 10, Con -, and ridiculously high mental scores? Maybe around 30in all 3, like the greater archfiends and celestial paragons?
The Hit Dice might need a boost to better fit their size and powers.