This is the one we now have. Linx 4 Modular. It's awesome. Our old one became unreliable and misfired often, resulting in studs that were not really welded.
The Linx 4 welds studs up to 5/16-inch. I think it also stud-welds deep threaded nuts. And I believe it works on 120V and 240V.
Anybody can operate one. You just insert a stud into the nose of the gun, press to the workpiece, and pull the trigger.
I don't remember the price, but we got a good discount by buying it at a sign show.
You can buy cheapies at Harbor Freight for a couple hundred dollars. I don't know if they work well or not, but I think they are more for dent pulling.
They are probably not suitable for sign shop work, but the Linx 4 works great.