Large external HD array for a Mac mini with RAID to run the Mac mini as a NAS
I have a 2012 Mac mini with a i7 CPU c. 2.5ghz....is there a way I could attach a large external drive array with RAID via USB 3 and run the Mac mini in Server mode to accomplish what i want to do without buying a Synology NAS ?


A "large" enclosure?  No.  The biggest USB 3 Direct Attached Storage device I've seen was a 5 bay unit made by Drobo (I won't have those anywhere near my data centers because of their reliability issues).  Can you expand them?  Sure, by purchasing another enclosure.  As long as you have free USB 3 ports, you can add another unit.  

Promise makes the Pegasus 3 Thunderbolt RAID enclosures that can hold up to 8 drives but they're horribly expensive.

So, (IMO) DAS really isn't a cost effective solution in the long run which, ultimately means the Mac mini isn't a great file server to begin with.  Additionally, macOS also lacks critical support for things like iSCSI making it a poor choice for a storage server.

I would go with the NAS as it's much more cost effective, feature rich, and flexible.  To the contrary, running macOS (an OS more oriented to the desktop rather than a server) just adds way too many layers of complexity to your storage equation.