Thanks guys, I used this thread for guidance when deciding the same thing as the OP. I'm well overdue having a NAS and moving my Plex server off of my gaming PC. I've not even heard of Unraid before but now I'm converted! The most appealing thing is being able to mix drive sizes with parity which means I've actually starting using an older 1TB drive again, because why not?
I ended up building the following:
i3 9100 with a Noctua NH-L9i cooler
Asus PRIME H310I-PLUS R2 Mini ITX Mobo
8GB Corsair Vengence RAM
Seasonic Focus SGX 450W PSU
250GB Nvme M2 (cache drive)
Fractal Node 304 case.
It cost me £450 before storage drives. Unraid is another £50 which means £500 for what is a pretty decent spec server that supports QuickSync with the iGPU so will transcode away effortlessly. That 9100 appears to be similar performance as my i7-4790k but without hyperthreading! If I need more than 4 drives, then a £20 Dell Perc card will allow me to add more (although my case can only take 6). If I wanted to trim cost then I would have saved £70 by putting a Pentium Gold in it but wanted the extra cores.
I can also say that Unraid was the easiest thing to setup ever, and must have taken me 30 mins to create the flash drive and get it up and running with some shares created.
Another advantage is that I can run a Windows VM if I like and I've actually got a decent usable PC. Will be sorting Plex etc tonight