I like this idea, but it seems a bit overpowered, giving you an anvil for very little cost. It would be better if it had more limits. My ideas are, that items that it makes should only be basic quality, so you would have to use an anvil if you wanted good armor and weapons.
Also, since sand, wax, and plaster can be used as casts, it should be no problem making stone casts. They would be made in a mason's workshop, and you would be able to make casts for many types of weapons and armor, as well as an anvil. There would be a new sub-menu added to the smelter, that would have a layout similar to the forges, with the basic item types displayed, i.e. armor, weapons, trap components, etc. When you selected one of these, it would show a list of all of that type of casts you had prepared. After a stone cast was used, it would be destroyed, the heat of the molten metal rendering it unusable for a second casting. It should also be possible to make iron or steel casts, that would be permanent, if you didn't want to set up a proper forge.
I generally like the idea of casting metals, and would want to see it implemented. Instead of being a temporary forge, before you make your first anvil, as has been suggested, I think it should be able to be made into an alternative to using anvils. If you wanted to save fuel, and didn't mind the lack of quality, you could have a set of iron casts made at the forge, then use those as an alternative way to make your equipment.