Here’s a link to a Roon/HQP Server I built a few years ago. You could certainly get more bang for your buck now. I used a mini-ITX form factor so paid a premium for smaller sized PSU and GPU which you can avoid:
My local music storage is partly SSD (1TB) and partly HDD (6TB). There is a very slight difference in startup speed for music if the HDD isn’t already spinning. Also Roon often rescans the HDD, but never the SSD.
Always put the Roon Core on an SSD.