Hi Paul
I built my resistance iron some years back - Maplin used to do a "transformer kit" which had a pre-wound primary, and a set of soft iron cores in "E" and "I" shapes. I wound the secondary from some very substantial varnished copper wire, fitted it in a box and put a big DPST button on top, which applied 240V to the primary.
I tried to wind the secondary to give three output levels and one common - actually, I almost always use it on "max", and vary the time - it lives on the floor and I switch it with my left big toe.
Electrodes are copper-plated carbon - I made a holder originally, but then discovered a commercial one, fitted with silicone insulated leads, and treated myself.
typical electrodes
Parweld Arc Air Gouging Carbons | Tools Today
I think the electrode holder is this one, the unit is a commercial version of my DIY effort!
Resistance Soldering Unit | London Road Models
usual disclaimers of course
Some other thoughts - solder a whopping great wire to the assembly, where the solder won't show, and use this as the earth. Also make up some brass clamps so if you clamp an earth onto a model, it doesn't mark it. Get two carbon electrodes for those jobs where an earth is not convenient or a pair of tweezers (google them - haven't yet got these myself) Sharpen the electrodes with a pencil sharpener. Use solder paint.
atb
Simon