A resistance welder is chosen to suit the combined thickness of the sheets and the access to the weld point. For car body panels and sheet from 0.6 to 1.5 mm, single phase 230 V machines rated 6 to 12 kVA are enough. For sheet from 2 to 3 mm plus continuous work you need a 400 V floor standing welder, usually 20 to 50 kVA, with water-cooled electrodes.
The second parameter is arm reach. Arms of 200 mm are enough for small parts, 500 mm and more let you reach inside a cabinet or a tank. The longer the arm, the higher the impedance of the circuit and the lower the current at the weld, so a long reach calls for a stronger machine.
Electrode force decides quality just as strongly as current does. Too little force gives expulsion and burn marks, too much flattens the weld nugget. On pneumatic machines check the cylinder and the valve, on foot operated ones the spring and the lever.
On welding guns for body work, what matters is a controller with time and current adjustment and the condition of the cables feeding the gun, because that is the part that wears out first. In the product listing we state whether the cables have been replaced.
Check the tooling as well: the set of electrodes, the tips, the puller. Copper electrodes are cheap, but unusual arms can be hard to source. In the listing we say openly what is missing and what it costs to complete.