Technically, you don't need anything but notepad to start modding. Textures and meshes will obviously require some graphic/3D modeling software, but you don't have to use them if you want to leave the graphics alone. That said, the modding community has generated some good programs to help you mod faster and easier.

Harpo's tool has some handy programs to convert the game files into the modable versions, generate manifest files for you etc.

https://forums.sinsofasolarempire.com/356807

This one has some more specialized programs to help with writing strings etc.

https://forums.sinsofasolarempire.com/358220