Both have their place depending on taste and budget.

By viable I mean workable, functional. A chimney sweep or a builder adept in chimneys will give you a report. Certainly enquire about a future gas fitting rather than logs if possible. I say this as a fireplace anywhere is delightful and, if downstairs, it could heat upstairs through the house, to a certain point too. If you look at the quality of the brick laying, you will see that its not meant to be a feature which is why it was rendered, but that is choice only. Certainly seal the bricks. Good luck and stay with it if you can. Everybody loves the romance of a fire.

Got my answer - non combustible material such as, natural stone - marble, granite or concrete etc.

Here is a photo of our distinctly unstyled fireplace. I move the plant and flowers off the mantel when we light the fire. It’s a different style to yours but all painted white. When I redo it, now the kids have stopped drawing on it, I’ll take the mantel back to wood. It did lighten the mood of the room but in the same white paint it lacks something. It you can afford it, do it. I’d recommend a closed wood box of you can get one to fit. Ours came with the house. It’s about 15 years old. No troubles at all (except the weekend a magpie fell down and gave child heart attack when it tap tap tapped to ask to get out) We have the chimney swept once a year. . It is amazing in winter. Or on a cold Mother’s Day.