Need help on ADC Building

Sivir isn't an IE core champion anymore; she builds Essence Reaver first and builds IE after her core just to supplement. It makes her Q poke hit like a truck and makes it far more usable without running the risk of going oom.

I suppose you could go Zeal first, but it really depends on your playstyle. I personally would never itemize attack speed without at least picking up a small amount of AD--a pickaxe or Bilge Cutlass at minimum. It's just not optimal imo because you're aiming to rain down large quantities of low damage hits rather than itemizing to make each hit count, and I would prefer to land one or two and back off than try and duel early game. It requires you to stay engaged too long to make it worth it because it leaves you open to gank pressure for a longer period of time.