A 40w Iron is plenty for 12awg wire...
A 100w iron will likely get too hot if it's not regulated.
My godfather swears by Weller and uses them in his Mercedes electronic repair business. I have a Weller 25w and a Weller 40w. The 40w iron with the larger tip will easily solder 12-10awg wire without a fit and they are very inexpensive. Easy to recognize because they have bright orange handles.
http://www.homedepot.com/catalog/pro...129742_400.jpg
You can likely find it much cheaper than this:
http://www.anticsonline.co.uk/533_1_17075.html
I bought mine for $19, or 11.40 for you guys.