You can easily run it on an i3. As I said above, I'm running it on a Raspberry Pi 3. Just bear in mind the newer gen will be more power efficient.
Can anyone please advise if an I3 processor or i5 could be ok for PLEX server? There are so many generations of above to choose it’s hard to know where to begin. Would an I5 generation 3 or 4 do? Or an I3 generation 3 / 4 / 5 / 6 do?
Sorry for the late reply, most good fan brands get their fans tested by various people; do a search for low noise fans, and you will find a lot of sites that review and rate them; often divided into sizes and price bands as well. Noctua make some of the best quiet fans, also very reliable, although not the cheapest. You have to balance how much air they move (CFM), against noise (dbl), and RPM. High RPM usually means more noise, but if it is a high CFM, you can run usually it at 50% and it can be very quiet, and yet still push more air than some of the "silent" fans.
Is there a way of knowing how quiet a fan is before testing? My current desktop sounds like a 747, but unsure what to replace it with?