I just moved Roon core from my desktop gaming PC to QNAP. I actually made a thread about it since everything went so smoothly. It’s only been up and running for little over a week but everything works great. I guess it’s still pretty unusual to run Roon on NAS, even though modern NAS processors are more than capable for the task. I’m not sure how it would benefit me to have a separate NUC/Nucleus running next to QNAP for the same task.
To answer the topic, I did some google searches about the same subject (best SSD for NAS) and ended up choosing WD Blue 250gb for the task. It was recommended as a budget choice. WD Red NAS drives start at 500gb and 250gb is plenty enough for system files. I have WD Red 4TB as a storage drive. No RAID configuration here but I do sync my music library to the desktop PC and take another backup on external HDD every now and then.