Some quick thoughts:
MIG : reckon with 40 Amps per millimetre of material thickness
Strength : for a tube or box fabrication, yes 3mm is OK, 5mm more than adequate, but for flat plate or angle iron, I would not trust a car's weight on less than 6mm
Choice of welder : a small MIG ( as above ), plus a 140 Amp inverter arc welder.
MIG : reckon with 40 Amps per millimetre of material thickness
Strength : for a tube or box fabrication, yes 3mm is OK, 5mm more than adequate, but for flat plate or angle iron, I would not trust a car's weight on less than 6mm
Choice of welder : a small MIG ( as above ), plus a 140 Amp inverter arc welder.