You actually may be able to fit three drives inside the case, along with the M.2 NVME SSD. So you could install three 4T SSDs, 1x3.5" and 2x 2.5", assuming you find the right size and quality drives you'd want for a NAS setup. Check out the Service Manual for details about where the drives can be installed in this PC. If BIOS is currently set to RAID, you probably will need to change that to AHCI before you install any new OS. Don't change that setting now because Dell installed Windows for RAID and the PC won't boot if you change that setting without taking prior steps to reconfigure Windows so it can use AHCI. BTW: If you have 12 GB of RAM now, that means you have 1x8 GB + 1x4 GB modules installed. That likely means RAM is running in slower single-channel mode, rather than faster dual-channel mode, which requires matched modules. So replacing 1x4 GB with another 1x8 GB may improve performance. You can see if it's currently running single- or dual-channel mode using CPU-Z (free) by looking on its Memory tab for the "Channel #".
Trying to determine the possibility of repurposing this desktop as a simple home server for shared storage.  It currently has a 512GB SSD and I would like to add two 4TB 3.5" drives.  I believe that I could add a small cage for the two drives, use a SATA power splitter and attach the drives to the two unused SATA connectors on the motherboard.  I'd plan to reimage the SSD with open source server or simple NAS software.   Any reason that this couldn't work?  Suggestions also welcome.
Inspiron 3671 add drives