Spot welding joins sheets without filler, using current and force alone. Three parameters decide the quality of the weld nugget: current, time and electrode force. Too little force gives spatter and burn-through, too much flattens the nugget and weakens the joint.
For a body shop choose a dent puller with a single sided gun and a set of electrodes for pulling dents and welding studs, washers and wavy wire. On modern bodies made of boron steel you need a medium frequency inverter with electronically controlled tongs, because a plain 230 V transformer cannot deliver the current density in the short time required.
For series production, arm reach and duty cycle matter. A bench spot welder with 250 mm arms handles sheet up to 2 plus 2 mm; for thicker material and stainless steel you need a 400 V machine with a welding power above 30 kVA. Check whether the unit has a controller with program memory, because on repeat work that saves setup time.
Electrodes are a wear part. Copper chromium tips need regular dressing, because a flattened electrode increases the contact area, lowers current density and weakens the weld. Keep spare tips and a dressing cutter in stock.