It's interesting that all the possible variants I had in my mind already appeared here.
I'm absolutely in love with Marina Diamandis's music and her image, still I think her voice would not suit Sally character, though it doesn't mean I don't like it, I love it, love it!
Paloma Faith is also a great option, her singing voice is right-alright for the character and she has that awesome vintage style that is hard to beat, in my opinion.
And someone named Kate Micucci - oh yes!! doubled.