MIG is the easiest to learn, and the best for doing sheetmetal.
Use Argon/CO2 mix and .023 wire.
The size of the machine that's right for you will depend on how much you intend to use it, and if you ever intend to weld "heavy" stuff bigger than ~1/8" thick. I have a 180 Amp (220V) Clarke machine that works pretty good... wish I would have spent more for a name brand machine though. ESAB, Miller, or Lincoln would be my recommendation. They'll hold their value pretty good if you ever decide to sell or upgrade. The "cheap" ones like mine have no resale value (which is why I own it... bought it used really super cheap
)