With MIG/MAG, first work out how thick the material is and how long you weld at a stretch. For sheet from 1 to 5 mm and occasional work, a 200 to 250 A unit with a four-roller feeder on 230 V is enough. For structures from 8 to 20 mm and continuous work you need 350 to 500 A on 400 V, water-cooled and with a separate feeder on a trolley.
The duty cycle is the most common source of disappointment. A 250 A machine at 60 percent will hold the current for 6 minutes out of 10, the same figure at 25 percent only for 2.5 minutes. For production look for 60 percent and more at your working current.
The second threshold is the way the torch is cooled. A gas-cooled torch up to 250 A is cheaper and lighter, but it heats up on long beads. A water-cooled torch works calmly up to 400 A and above, at the cost of a cooler and coolant.
Check the feed rollers: a V groove for solid wire 0.8 and 1.0 mm, a knurled groove for flux-cored wire, U rollers and a PTFE liner for aluminium. A steel liner with aluminium means the wire will jam.
If the machine is to run self-shielded flux-cored wire, make sure it has a polarity switch. In our listings we state what a machine has and what it does not. We name anything that is missing openly, together with the price of adding it.