For modelling.
Milkshape - Easy to learn, $35. Limited features and uses.
Blender - Harder to learn, free. Many more features and uses.
For scripting, use any text editor. I use Notepad++
And you need to be able to script in order to get your addon to work.
'Scripting' a weapon, in most cases, is just changing datablock names and variable names. It doesn't require much knowledge.