It's hard to say anything useful without details...
I'd strongly recommend hiring an engineer to advise you on this. Preferably one who works on laboratory safety, since that's the closest thing I can think of to your scenario.
But as a non-expert reaction, which you should absolutely not trust:
Plaster has a higher fire rating than you might expect. There's water bound into its structure, and evaporation as it breaks down will delay the fire's progress a bit. Not much at these extremes... but one of the ways they make cheap dicument-protection containers (often called "fire safes", though they are generally not good security devices; "fire chest" is a better description) is to use a thick layer of a cement designed specifically for that purpose, and a good one is typically rated to keep its contents below 450 degrees F for an hour in a 1700-degree F environment.
So enclosing the room in multiple layers of suitable plaster/cement might help isolate it in case of an emergency.
Re fire suppression: Again I'd talk to an engineer. The non-water professional solutions (labs and industrial kitchens) involve filling the room with nonflammable gasses like Halon (tm?). These are not toxic, unlike CO2, but that won't stop them from smothering you, so there are serious safety issues here. Such systems require recertification on a periodic basis.
Which reminds me that you may need to look at active ventilation and heat extraction. And making those work with the fire protection.
This sounds like something you can't afford to do wrong, and may not be able to afford to do right. You may want to reconsider the project, or at least the approach, if you aren't able to make the needed investment in manhours, materials, and equipment
Then again, people do sometimes try to weld in their basements, which would present many of the same issues. If there is a small single point at high temperature, without much thermal mass, and with suitable safety precautions, it might not be as completely unreasonable as it seems. (But people also burn their houses down with much lower temperatures...)
But If you need to ask, you need expert advice, not a bunch of people in the Internet. Seriously. Engineer who knows this topic.
As far as fire PROOF goes: nothing will ever guarantee that unless it's constructed from the ground up with that as part of the design. See previous; I don't think your budget will handle it.