All,
time and again the question of suitable hardware is asked. Because I just changed my hardware, I would like to put my two cents in. I have no special requirements for the hardware. I wanted to connect my WD-RED SATA HDD and the server should be energy efficient. With the following hardware, the server consumes approximately 5(!) watts in idle mode.
- D3643-H Mainboard by Fujitsu (appropriate BIOS version supporting CoffeeLake-Refresh processors required)
- Intel Pentium Gold 5420T (T-Model is probably not required)
- 16 GB RAM (be aware of compatibility list: ftp://ftp.ts.fujitsu.com/pub/Mainboard-OEM-Sales/Products/Mainboards/Industrial&ExtendedLifetime/D3402-B_D3417-B/Documentation/Specification/)
- 256 GB SDD (be aware of compatibility list)
- 3 TB WD-RED SATA HDD
- PicoPSU 90W with an external power adapter
- Dynatron K5 Topblow CPU-fan
- Noctua NF-A8 80mm PWM housing fan
I’m on Ubuntu 19.04 (for testing purpose only) with no screen, mouse or keyboard connected. All I´m using is
sudo powertop --auto-tune
and hd-idle for the HDD. A small instance of openhab is up and running well 
My RPi 3B+ with an external SSD and openhabian needs approximately 3 watts. So I´m happy with my new server and will continue to set up more services (media-server, nas, etc.).