Ya I had to read that passive like 3x and it's still confusing without actually playing him yet. It's the same shit they did to MF a while ago. Hate that.
Anyway, the aim seems to be to prioritize Atk Speed+Crit on him since those stats give +AD. And what you also get is Movement Speed boosts every time you crit. So it'll be interesting to see if his build actually differs from any other ADC champion. The downside is his crits don't do as much damage and attack speed doesn't make him shoot faster.
He'll be a positional ADC and that movement speed boost will be critical in fights since he won't be able to lifesteal up very effectively. I'm not even sure that he'll be a bot lane ADC. Seems more like mid-laner to me. He's like an AD caster/sniper. The sniper archetype hasn't really ever been explored before in League, so it's kinda cool.
Regarding items though, seems like he'll be very similar to Vayne and prioritize Zeal first then into whichever: Shiv/Rapidfire/PD most likely. Probably needs BotRK, but the lifesteal is kinda wasted. IE is also not very efficient since the passive won't be as effective. Maybe just prioritize Zeal weapons, just like Vayne does. Something like: Shiv > Rapidfire > Berserkers Greaves > Phantom Dancer > (Lord Domik's/BT/Mercurial/BotRK/etc)