Sadly, your ship has sailed here :-(
IFIXIT at one time offered a upgrade kit: [https://www.ifixit.com/Store/Mac/Mac-mini-1-TB-Upgrade-Kit/IF107-070|Mac mini 1 TB Upgrade Kit]. As well as the cable alone.
This system is so old I don't think it's wise to put to much into it. A SSD upgrade while nice maybe difficult as the SATA port of this system is only SATA I (1.5 GB/s). Make sure the SSD you get can run at this SATA speed. Also make sure your systems firmware is up to date first.