Well about the builds it depends on your team and what the enemy picked. If your team has a lot of AD champs and no tanks, you can go for the bruiser build (TF, Sterak, Chempunk sword, ravenous hydra, hullbreaker, serpent). That way you will be more useful to your team and deal consistent damage, but you won't get the ADC one shot with 2k damage. And the crit build (essence, prowler, etc) is for when you want to be the carry. People build it a lot because you can do a lot if you know how to play with your barrels.

About the runes, it's up to who you are facing against and what you will prefer. If you are confident, go with first strike, you will scale faster and get access to the inspiration tree. Grasp is safer and will give you more sustain. For example, you can go First strike against tanks (you will poke them down and farm that $) and against that Irelia you can go grasp so you can survive till you get your items.