Do I need anything else? How will it link to my iMac - where the files are? Will the Imac still be the control - ie the iMac is connected to my DAC. Or will the NUC be connected - if so what is the best connection? (connecting to a CHord ExQute)
I would use an M.2 ssd perhaps something like this, it is higher performance than the sata ssd you listed, also you may want to consider a nuc7i3 or 7i5 as they run much cooler and therefore have less intrusive fan noise than the i7, all depends on your library size and what you are interested in doing with regards to upsampling etc.
Files wise I would put them on an appropriately sized usb drive attached to the NUC.
To attach to the Chord you can either connect directly to the NUC (another reason to be careful on the choice of NUC to avoid fan noise especially if you will have it in your listening room) or use an Ethernet to usb bridge such as a RPI or Microrendu.
Your Mac will operate as remote control whether it is the core or the NUC is the core.
As Slim_Fishguttz says, stay with the Mac… decisions decisions.
A couple of things. The first probably isn’t essential but I would purchase 2x4 gig sticks rather than 1x8 gig. Probably not essential but worthwhile. Secondly I would put a M2 NVME stick in rather than the SATA SSD. That leaves the SSD space free to use if you wish, and NVME is faster than SATA which will be reflected in Roons performance. All of that said though, what you have chosen should still work just fine.
The SSD needs to be there for Roon’s DB – that’s it. On MacOS, that’s in $HOME/Library/Roon
You have to create a symlink from that dir to your SSD if you want to not upgrade your OS drive. This method is pretty technical and requires an understanding of mount points + basic UNIX command line skills. I could give you the commands to run, but if you have a problem later, you will suffer. I suggest just upgrading the OS drive for the least trouble in the future.