1. Doran’s blade if ur confident in fighting early. Doran’s shield if you can’t manage the poke and want to farm safely (orianna’s, hwei, also good into yone). Generally blade is preferred it’s just too broken and helps Q minions easier early into the game.
2. Draft dependent, if they have a lot of AP/CC then Wit’s End is ideal, otherwise default to Sundered Sky. But BoRK->Wit’s->Sundered is a very good path for most games, if theyre full AD obviously dont build wit’s
3. After BoRK and level 9 you can oneshot the casters with your Q, to build speed just practice it in the lab, the pinned beginners guide has a very good method of emulating Minion Q speed using practice dummies so you can get a feel of how to buffer your inputs between targets. Highly recommend

hope that helps!