I’ve also evaluated buying a parts a built it from scratch, using like the new intel n100 with is very powerful with low power usage, a big case and so on for a similar price.
Yes do that. You wont need a very powerful CPU. Consider that you’ll saturate your gigabit ethernet before your CPU when using SMB thus it doesn’t make sense to have all powerful hardware. Another important thing to consider is the software: if you go bare Debian fully headless / no desktop environment and install the things you need manually - avoid Docker overheads, TrueNAS and other useless overheads - you’ll use WAY less resources and your system will run considerably quieter.
Here is a real-world example with am i5-7400 + 8GB RAM + 20 TB BTRFS RAID 1 NAS I built:
While transferring a big file from a Windows machine the NAS CPU is handling it just fine while the network is already saturated. Btw I got that CPU+Board+RAM for 70€ second hand.
The only thing to consider is that if you for plex you’ll most likely want a CPU that has good support for video decoding. Do your research on that but believe me with I say under gigabit speeds and mechanical hard drives ANY CPU will make it.