I like the liberal arts studenty style I fell into just as I was leaving university: colours that remind me of a stormy English countryside landscape (indigo, oxblood, cranberry, aubergine, charcoal, forest green, stormcloud grey, silver grey, navy, etc), slim fitting trousers and blazers, a-line dresses and skirts, Dr Marten boots, subtle coloured tights, too much silver jewelery, and as much 1990s inspired stuff as I feel I can get away with.
I buy my everyday smart-casual clothes quite cheaply:
Straight-leg, medium rise cord trousers in navy and cranberry from M&S;
Straight-leg black denim jeans with lots of stretch (I get mine from Sainsbury's as they're the only kind that fit properly - YMMV);
3/4 sleeve, scoop neck linen t-shirts from Zara in dark colours, for layering;
Lace trimmed vest tops in dark/muted neutrals from Primark, for layering;
Above-the-knee tube skirts in dark block colours or stripes (the Hush ruched one is nice);
Oversized grey denim shirt from the men's section of All Saints (used to be DB's until he grew out of it), worn open over a vest top;
Cowl neck jersey tops in black and dark purple from H&M, for layering;
Unfussy, longline, jersey waterfall cardigan in black (from Tesco years ago, but I can't find any non-frumpy replacements in the shops now);
Silk velvet blazer in black (boutique designer, bought on ebay for a song);
A line leather mini skirt in black (again designer, bought on ebay very cheaply).
With a bigger budget and a different occupation, I'd then save up/use the left over money to get some really beautiful smart dresses, silk blouses with open collars, and a perfectly fitting pair of black dress trousers from more expensive shops. That might not apply to you - I have to because I have a really awkward figure to dress for formal situations (short, very small waist and good legs, but GG/H cup boobs and wide hips).