Hi Mario
We have created a 2-way integration for Magneto which support NAV 2009, 2013 and 2015. Making the as simple as possible on the NAV side. However you also need to update the Magento solution to support the requested features however we also have that.
Our website (www.viincera.it) has not yet been translated to English, but feel free to write me an email (mcc@viincera.it) or give me a call (+45 5352 0001) then we can provide you with the if you need.
PS our integration also works for Dynamics AX and Pyramid but most likely also for other ERP systems as well ;-)