I think it's shoes that make ankles look thick, not trousers, iyswim. But one of the reasons I like those ones I linked to is they're a good stright cut, not too tapered, so won't cling at the ankle, which is flattering, yy. And if you like them, then try the black! whether black is a good neutral for you will depend on your colouring (eg it can be harsh on fair blondes).
If you want to up the chic / glam ante a classic pair of courts really helps the legs but a decent pair would take your whole budget (and it's worth spending £100 - £150, I did on a pair I loved about 8 years ago, have taken care of them, they're still fab).
If you can ditch the DC (ha) ideally go and get some trousers then pop them on to try shoes on. I know, who has time for that.
Nails - as long as you avoid novelty and they're neat and shortish, I don't think you can go wrong. Nude hands are trendy at the mo. Bright pinks, reds and corals are always nice on toes, summer or winter. Though just think about the combo with your outfit - eg I would find red toes and black trousers a bit vampy, but red with navy, grey or khaki clothes is brilliant.
Sorry if I'm being well bossy, I'm supposed to be revising for exams and am finding it far nicer to direct my energy elsewhere, whole afternoon wasted on MN 