I'm pretty sure it will be a fibro cement (non combustible wall) needed rather than gyprock/plaster & the fireplace will generate the bulk of the heat from the flue. As long as correct distance (building code will dictate) is maintained to combustible surfaces there should be no problem. Have you considered tiling the wall?

I'd presume the combustion fireplace is a built-in unit. I remember ours heating the bricks up quite well. I don't see an issue plastering the brickwork but look to use 'Fyrechek' which is a fire retardant plasterboard. The bricks could also be rendered instead but I reckon you'd be able to pick the different surfaces once painted. I wouldn't think you'd need a heat resistant paint. The walls would be warm but no warmer than a wall copping direct summer sunlight.

I just came up with this it is very hard area to work with what do you think about this it's just a rough plan my house hasn't got a front door only in the garage

.are you putting another vanity in? I d remove the cracked tiles and retile with same size tiles if you r planning to paint iles