This combat walker started life as the Knight resin kit from Industria Mechanika, a beautiful resin kit in 1/35th scale. It is an awesome kit! The legs, the arms, weapons and torso are all positionable. It comes with a full cockpit but I built mine as a robot.
I began by adding on extra armor in places I felt a real walker would have it, then scratch-built a larger lower torso to make up for the extra armor. After that, I scratch-built the rail gun (with the kit you can arm it with either two rocket pods, two gatling guns, two 120mm cannons - or various combinations of those.) I had to scratch build the feet, as it seems my puppy thought they tasted good. I also added hydraulic accumulators in the legs and the wiring and plumbing.