Gundriver

A young man named Dick lives a peaceful life on the frontier with his father, as well as his sister, Anna.
Dick wants nothing more than to be able to buy a Gun Driver, which are large mech suits piloted by gunslingers. However, despite saving up enough money, when he goes to buy one he is turned away due to racial prejudices. Upon returning home, his world is turned upside down when he comes face-to-face with a gang of Gun Driver pilots called "The Wild Bunch", who attack his home and leave him without the use of his left arm. However, his life is saved when his father's left arm is transplanted onto him, allowing him to press onward. He then joins a mobile brothel called the "Jewel Palace", who will aid him on his quest for vengeance, and in rescuing his sister Anna.