Might not even need the traditional sniper rifle... I find it very rare that the player's or NPC's are at any range that requires longarms most of the time. In fact I would think a smaller weapon would be better as it is easier to find a closer position then it is to find a long, unobstructed view, free of clutter, buildings, signs, ect... It also makes your escape easier when you simply pocket the weapon and walk off blending into the crowd (not to mention easier to conceal around town). Double tap those pistols, make a ranged throwing adept, there are lots of ways. But if you wanna stick to traditional sniper trope, that can work too.
If you take physical adept, some powers to look at would probably be wall running, traceless walk, taking the invisible way, improved skill ability, hang time... there are many ways to be honest. You could take face powers to mimic, morph, or vary appearance..