First buy? An oversized army shirt or jacket. You can wear it with all your summer frocks or jumpsuits like Alexa here, and over a pair of jeans when it gets cold.
Khaki is going to be huge - the colour of the season - and you get extra points if it's bonafide military (plus it'll be way cheaper). It's a great way to extend the life of your fave summer outfits and it's not exxy.