I put metal siding on a house last year and the metal cost less than the vinyl would have. I wouldn't have thought that possible. The fabricator was local so no doubt that helped. It would be worthwhile to check into what's available to you locally. It will take time to research but it may pay off well.

If the brick is a functioning chimney I would leave it other wise a rough render could work. One of the issues is that the nearby features such as the light and painting clash. As in the dining room, try a large mirror and a big photo e.g. to link it all. Also good rugs with strong reds, browns, oranges would help co-ordinate the rooms at low expense. I would try a colourful Moroccon style rug near the brick feature before attempting a major fix.

I would suggest a stone hearth matching the plain stone trim but not as wide as the whole unit to reduce tripping hazard. Possibly with a black trim edge. Then a mantle shelf. the same stone would be great but probably too heavy. I think a 'live edge' timber mantle would be great and introduce an organic edge to the straight lines and angles. then paint the flat surface a neutral colour as others have suggested. display something with white/cream tones (candles?) and something with an accent colour on the mantle. if mantle too pricey a big artwork a little wider than the fireplace itself with some black in it or a black background chinese screen or similar. I have no commercial affiliation with them (just like their work) and don't know where you are based but Time 4 Timber Brookvale NSW could probably help you with a live edge shelf/mantle. good luck!

if the fibro is in good condition, simple paint and enjoy your gorgeous vintage home if there's any damage you can replace the old fibro with a modern cement sheet using simple timber battens (looks like the old curved fibro cover strips) and making sure to add suitable insulation...the advantage of using similar thickness cladding is there's no expensive alterations needed around windows and doors although, if you decide to remove the old fibro it's quite expensive to have the professionals do a big job but check the latest regulation and you may be able to replace small sections at a time