I recommend the maletrics arduino spotwelder, cost is 75 euros (93 dollars) just for the welder, you have to buy the probes seperate. I been using the older version since last year, the newer version is a major improvement but cost the same. He sells it off his malectrics.eu website, from germany shipping I got it in a week to california.
I built my own probe cables with 6 gauge pure copper welding cables, you need that thick copper to get all that amps into your probe tips. I use 015 nickle and get good welds at the low settings, the welder has plenty of power to spare. I been using a small 12 volt 28 ah agm battery to power it. The newer version you can use a 3s lipo pack to power it.
I heard too many reviews of the plug-in models blowing fuses, thats why I recommend the battery powered ones, but if you got good electricity thats an option. The battery powered ones might not look impressive because of there small size but they got plenty of power to get the job done.
With whatever tab welder you get, don't go cheap on the probe cables, I see many people using the copperclad audio wire for probe cables, those cable will get hot during using and they will jump around alot every time you do a weld. With the copperclad wire you will have to increase the power output of the welder to get good tabs, thats why the cable get hot. I was using 8 gauge copperclad when I first got the welder and the cables were too hot to hold after about 20 welds, with 6 gauge pure copper you can start and finish your project without stopping to let the cables to cool down.
At the endless sphere forums, they got some good threads on some of the different arduinotab welders. Another plus for the arduino welders, they don't take up much space.