blender, it´s free

>
http://www.blender.org/
It´s recommended to read the blender documentation to learn this program, it got´s a really huge field of functionality and possibility >
http://wiki.blender.org/index.php/Doc:2.6/Manual or watch youtube tutorials, there are a ton of it, or hop over to the maniapark tutorial section, there are also some helpful things >
http://www.maniapark.com/forum/viewforum.php?f=14
For textures, use the ingame textures (list can be found in the nadeo importer material lib) or if you want to make your own, you can use gimp for that (it´s also free) >
http://www.gimp.org/
For editing the xml files, it´s recommended to use notepad++ (also free) >
http://notepad-plus-plus.org/
And last but not least the nadeo documentation and the importer (download the sample files also) >
http://doc.maniaplanet.com/creation/importer/