A) I've seen a lot of people recommending NUC100 computers, but the sata ports may be limited.
I currently have my old computer doing the thing (an i5 6500) with Unraid OS, really happy with it BUT they are changing their paying model this week so I don't know I would have went this way. I don't like subscription models.
B) Why not both? I have two SSD of 512Gb for items currently downloading and two HDD for Capacity (each 8Tb) already 30% filled in 1month. Both set of hard drives are set in duplication mode to avoid losing any day.