Hi Umberto, welcome! I hope that MoI will be useful for you!

If you're new to NURBS modeling but have prior experience in polygons, there are some tips here for people coming from a poly modeling background, they may be useful:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4865.2

The strategy for NURBS modeling is very different than for sub-d modeling, you want to form a lot of your model from 2D profile curves and using cutting/boolean operations as the main way to build things. This process works particularly well with mechanical man made type objects, once you get a little used to the different modeling strategy those kinds of models can come together very quickly with NURBS modeling.

Thanks, - Michael