All of the responses here have pushed me towards a MicroServer. Based on the responses here, I plan on getting something like the HP MicroServer or the Lenovo ThinkServer, with an Intel CPU (in case I want to use Plex). I was going to go for 2 x 12TB drives which would have cost too much, but realised that I can go for a RAID 5 or 6 with drives (probably WD Red or Seagate Ironwolf) around 6 TB each to give me up to 24TB. Then all I would need to do is get a suitable backup drive which would be kept separate from the NAS. I assume that I can use unRaid to mix drive sizes? I like the sound of having an SSD cache drive, will look into it further.
At the other end, I'll look into a range of devices like Pi, Firestick, chromecast, smart tv's, etc. Are there any recommendations? Need something that has an easy to use interface, can handle the quality of 1080p and above, and can display subtitles from SRT files.
To keep costs down, I'll look at getting used items. As with any used item, I do run a risk, especially with the drives; so are there any reliable tools to check the drives? Any pointers on which models or server I should go for and which I should avoid?