To make tools, you should just need notepad; tools don't really need models or textures.
Models: Any program can be used, but ones that are able to export to .dts help a lot. You don't need to constantly export to different stuff to get it to .dts.
Decent ones that I know of: MS3D, Blender, Wings3D ( Can't export directly to .dts ), GoogleSketchUp ( Can't export directly to .dts )
Scripting: Notepad, or any text editing program. Notepad++ is pretty nice if you want more features.
Textures: For Blockland, there isn't really a need to put special textures on models, so you could just use paint and use one colour. Although, if you want to make things like decals, any image editing program is nice.
Decent ones that I know of: Paint.Net, GIMP