Their weaponry is military-grade. As for using a sledgehammer on them, that depends on how strong the person behind the sledgehammer is. They'll take twice as much punishment as a normal human, but if you kill them gruesomely, well -- their eyes are half metal, there's a giant hunk of metal machine in their chests, and some of them have metal limbs. Wires run down their organic limbs, as well.

Finally, yes, there would be: to the south of the Dead District is a low-income area where a lot of the gangs do their thing. LSR clinic is also there.