I'm using an AMD Turion and running ~6 VMs on that. I used to run pfSense on it as well, but I had to take out the second network card to put in a HBA.
You will moust likley run out of ram before you hit the 80%cpu utilization even with 16G of ram :) Also all the services you said are perfectly fine and you will even have room left for more services
Just be mindful that for Plex that chip has no iGPU (unlike some Xeon CPUs) so will not have QuickSync for better hardware transcoding. Also Xeon class systems tend to consume more power and electricity bills are a little higher than they were over 5 years ago. Having said that, I've been running a Kaby Lake based NAS from brand new and it's been a great little workhorse, with over a dozen containers running 24/7 sharing only 4 threads!