Engineer or Warrior?
Go warrior.
He has traits that support both the Longbow(you’ll want it for AOE)and the Rifle, which work well together for the longrange sniping gameplay you want. Also, while I don’t have screenshots, Heavy armor is pretty varied and should allow you to find some decent “sniper” armors. (In particular, the HoW armor is nice)
Trait lines you’d want to focus on for warrior are the Precision and Vitality Trees. Precision has a trait that makes rifle shots pierce (!!!) and reduces their cooldowns by 20%. The vitality tree has skills that make your longbows shoot from further away, and do more damage to bleeding targets (And rifles inflict bleed. how…. curious.) From there you can put more points into power for a direct-damage build, or go condition damage to facilitate the burning/bleeding that the rifle/bow does. It’s really up to you at that point.
Basically, what I’m sayin’ is, if you want to be a Sniper, go Warrior.