Replacing a MAC Mini with new one...

You don't say why you need to replace the Mini.  If you are happy with the current setup, then replace it with exactly what you now have.  Used Minis are plentiful and relatively cheap.  You could even swap the hard drives between the old and new to avoid any changes in configuration.

Mesch, nearly everyone would recommend that you store the music files on a separate hard drive than the system drive.  The 500Gb hard drive on your current Mini is more than sufficient for your system files.  Alternatively you could install a smaller SSD drive for an increase in loading times, less heat, less noise and greater reliability.  An quality external 1Tb drive for music files with a second one for backup are available for under $200.  8Gb of RAM is nice, but 4Gb is sufficient.   What I'm saying is that your current Mini with some additions may be all you need.