MIG welding can be used to weld steel, stainless steel, and aluminium. Each metal will require slightly different gasses, welding wires, and current settings. Furthermore, different thicknesses of the material may require larger welders. The heavier and thicker the metal being welded the more powerful welding machine needs to be.
For home MIG welders, a machine rated between 140 and 220 amps is most likely the best choice. This would allow working on metal thicknesses from 1/16″ up to 1/4″ including pipes, tubes, flat bar, round bar, angle iron, sheet metals and square hollow sections.