https://www.houseandgarden.co.uk/gallery/farrow-and-ball-colours
I found this article really helpful for envisaging how to use colour, but also there are common themes that you could explore for adding character and charm. I'm no expert but would start by looking at colour (agree with pp that chalk based paints are good for creating character), layering textiles and replacing 90s radiators (or adding covers), doors and skirting. Some proper wooden cabinetry, painted, will add interest, colour and depth, and if this includes book shelves the books and other interesting/attractive objects will do the same. Lamps rather than ceiling lights will soften edges, replace fitted carpets with wooden floors and Orientals. Curtains or roman blinds that are interlined give a sense of comfort and again, soften the edges. A little clutter helps- smooth clear surfaces are the death of charm. And really good antique furniture is incredibly affordable right now, especially at auction, and a great short cut to making spaces feel lived in for ever. Lots of art on the walls, it doesn't have to be expensive.
90s houses often have lots of cherry-stained wood. You need to lose the stain as it's so period-specific. More paint!
What cupboard fronts do you have in your kitchen? If they are modern looking could you replace the doors with painted wood? There's a current MN thread 'Can I see your kitchen' with lots of ideas (you'll see more from @LibertyLily there).
The single biggest thing is probably a working fireplace- do you have one, or could you retrofit one? It would eat a chunk of budget but make a huge difference.
It sounds like I'm spending way over your budget, @Dominey , but so much can be sourced at auctions and on Etsy or eBay etc, much cheaper than new purchases and with ready made patina and eclecticism.