QSS or GA are amazing adc items. If I recall correctly, QSS gets rid of Exhaust and other debuffs, ultimately making it better than any damage item you'll be able to pickup. If it doesn't get rid of Exhaust then I'm sorry for misleading. If you're just 100% committed to damage, a stacked Bloodthirster will give you the most output, Frozen Mallet probably the most uptime on champions. I've wondered about getting a Trinity Force on her too. Black Cleaver can be a nice addition if you feel the other team is super tanky and need more ArP but I'm not quite positive how attack speed stacks/falls off.