Ok, I'm mainly thinking about buying Corki. I just farmed up enough IP to buy him, and i just want to be sure it's a good choice before i actually buy him.
As of right now i mainly play Ashe, and do own Tristana. I am decent with Ashe, but am pretty bad with Tristana. So far I have played Corki for a grand total of 3! times, and I liked him overall. I would've played some more, but the champion rotation changed and i missed my chance. I do know that there are other ranged champions i could look at, but i haven't really played anything besides Ashe so i can't really judge anything.
As for how i build him, i would go AD and i have ar. pen, crit%, and crit dmg marks. Im level 25 in game, so i don't have all my slots filled but the ones that are open are filled. So if i do buy Corki I will be left with around 50 IP, and won't be able to buy runes till i get some more wins.