My Mac mini runs headless 24/7 acting as my WeatherCat Server, my VPN Server and my FTP Server.
The original was a second hand 2011 model that did the job well but was sluggish when connected by screen sharing from my Mac. Also it would not update to Mojave as High Sierra was the last version of Mac OS it would support.
Due to its age I decided to preemptively upgrade to a new 2018 model.
The one I chose was a 3 GHz Intel Core i5 with 16 GB or RAM and a 256 GB SSD.
You may say a bit high specced just to run a few servers but I expect it to be in service for a good many years so just "future proofing"
It runs like a dream!
If anyone is interested (and didn't know) Mac OS has a built in VPN server and can be activated from the terminal with command line commands. However I used this as it is much easier to configure.
https://cutedgesystems.com/software/VPNEnablerForMojave/There is a free version for High Sierra
https://cutedgesystems.com