Looking to find out if it will be up to code, and what kind of things I should use. Cement board? instead of dry wall? insulation?
Last picture if the room above the fireplace.
Your request is not really clear so let me guess?
You want to put a cupboard above the fireplace or you want to replace the existing structure with a cupboard?
-the fireplace has a rated vent pipe, insulation etc. above it, so "no" you cannot put a cupboard there.
-you want to replace the entire thing with a cupboard? The fireplace chase in main room is not load bearing or structural so you can just rip it all out. The gas line needs to be cut and properly capped by a plumber/gas fitter. Any electrical, same thing by an electrician. The vent pipe needs to be cut wherever you do not need it any more in new space (likely at the ceiling inside) and insulation shoved inside. The pipe and wall upstairs can be left as is to save you money. the vent cap on roof is rain proofed but some more insulation shoved down at top afoot or so will not hurt either. Then do your cupboard.
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