Darkbrady's link to guns has good information on what guns to use.

Armor is depending on the defense you need if you are talking PvE.

Nameless is preferred for FSC -> Elemental for Vana with ASI. This is also preferred for any elemental dealing level. The medium ASI helps.

Justifier is Nameless just piercing defense instead of elemental defense. I would get this and Nameless, because of things like SL Snarbolax / future piercing levels that aren't a cakewalk. It also makes sense to switch to piercing defense for things like a slime / beast arena which both have polyps (usually at least).

Deadshot is shadow defense gear with a medium damage to undead. Med damage to undead. It helps in FSC if you have trouble with zombies but you should not. This would be used in things like Graveyards / used as your shadow defense set. This armor is definately not necessary though. Skelly, for example, has greater shadow defense and poison and freeze status def (both appear more often than curse) while sacrificing medium damage to undead. I like to couple my Divine Veil with Deadshot Mantle to have max damage against fiends and undead for dark cities etc.

Shadowsun is Justifier, just negative poison and negative curse and swaps medium ASI for low damage. This set is the only way for you to max out damage against everything, but it is only low damage per piece. Negative poison hurts in SL Snarbolax. Personally, I would not get this and go for Justifier instead.

And for trinkets, get two damage bonus trinkets.