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The true artisans of old-car restoration tend to sneer a bit a MIG welding, preferring the (largely lost) art of "hammer welding". With hammer welding you use an Ox-Acetylene torch to weld and a set of body hammers and dollys to correct the distortion. There is a technique to welding "a little at a time", and using the hammer as you go. You can buy video tapes to show you how to do it, or look at various websites. This method has two big advantages -- the needed investment is smaller if you already have the tanks and regulator, and the welded areas retain the same material properties as the surrounding mild steel, better facilitating hammer forming of the correct shape and contour in the finished product. Mig welds are too hard to hammer form. Interestingly, production body shops sneer at torch welding for two reasons: 1) It is unacceptably slow going for production work.
2) Quite a few automotive "crash parts" these days are built out of heat-treated high-strength steel. The large "heat affected zone" of torch welding will anneal the steel and render it back to soft mild steel, leaving the repaired area weaker than the surrounding metal. The typical ER70 MIG wire has similar material properties to the high-strength steel, making it a compatible repair. If the areas you need to weld are flat, the MIG welder will be quicker and easier to learn. If the areas have compound curves, like the tops of the fenders, then hammer welding would let you massage the metal into the correct final shape, and use less Bondo.
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