I bought a MIG welder because it has versatility and is easy to learn. After practicing a bit on scrap metal, I'm about to do my first welds on the Alfa.
MIG welding sheetmetal using butt joints is not difficult. As mentioned, it does take some experimenting to find the right settings for amperage, wire feed speed and aiming the gun.
It's important to carefully trim the metal that is being welded together for a close fit. Also, the two panels must be thoroughly clamped so that they end up flush when the weld wire gets shoved at them.
Bill / 1977 Alfa Romeo 4C2000 / 2012 BMW S1000RR / 1975 BMW R90/6