To model you need 3dmax wich costs more than 800$.
To skin, everything you need here.
To edit the graphics, you have THE GIMP ( Tools section ) or you can get Abdobe Photoshop, that costs a hundred of $ too.
Then You'd need War3 Image Extractor ( Tools section ) to:
1. Find a texture
2. Save it as a .tga
3. After you edited the .tga in either Abdode or THE GIMP, you will open the edited .tga with it.
4. You will convert it back to a .blp
And of course, a useful tool that is only optional: Wc3 viewer 2 ( makes you view your skin on a premade wc3 model without having to start war3 )