The ebay item looks nice. You might also want to check out Shapeoko, Shapeoko | Precision by DefaultShapeoko | Precision by Default and the forum there, as well as eShapeoko MakerSlide Europe - MakerSlide Europe - (Powered by CubeCart). I don't think you can do without machining at least some custom parts, but you can design a machine so, that you only need to cut sheet metal. Cutting and drilling aluminum might be doable for you? Even if not, sheet metal parts should not be very expensive from a laser cutter shop. Gerber is not a good file format, but there are some free or cheap mechanical CAD programs.