Maximum PC

HI,
Looking for some advice
In my current system I have 6 HD's of different sizes and speed and interfaces (sata I, II, III) I have 2 optical drives, and a multi-card reader. Some HD's have several partitions. My drive list in Explorer is very long.
I have 3 other computers on my home network with large single HD's with 2 partitions each (1 is OS and programs, the other is data and backup), 2 are wired, and the HTPC is wireless. Most drives are about 1/3-1/2 full. except one that is near full with games from STEAM and is one of the wired computers.
I was thinking of building a NAS. I have never used a NAS before. But I have enough parts laying around to easily build one, except for an enclosure.
Each computer has it's own Itunes library. And everyone wants to keep those music files separate. But we do want to share pictures, and movies in itunes, and see them on the HTPC.
I was thinking of pulling all the HD's from the other 3 home networked computers (and extras HD's in my main system) and replace with smaller SSD's for the OS's and office, antivirus, etc.
The other users know nothing about networking drives, mapping drives, etc. They need their data to "just be there".
Can a NAS help with these issues? How to set up Win7 for each computer? Will I need addition SATA cards in NAS to connects all these drives (9+)? Will the NAS storage just look like a really big drive? Will the dis-similar drives cause issues with RAID mirroring?
Thanks for your comments