I built a system ground up with a focus on power-usage a few years ago. You can go far down the rabbit hole when you start googling, but the key take-aways for me are:
Eliminate features you don’t really need. E.g. do you realy need a RAID 10 configuration with 4 disks? Or can you get by with 2, or maybe even 1 (which might mean you experience downtime while waiting for a new HDD, but you do keep backups right?).
Standard 300-400w ATX power supplies are inefficient under low loads (meaning much AC goes to waste converting it to DC). Use a PicoPSU (or a motherboard that runs on DC). Search for a high-quality power adapter (e.g. Leicke) that’s rated for low loads. Just make sure your PSU can handle booting, when your system will draw much more.
Install drivers for motherboard features you don’t need (like soundcards or wifi) and then switch them off in the OS as this usually sets them in a more power-efficient state rather than disabling them in BIOS.
SSD’s use less power than HDD’s but get expensive for higher capacities. I use a Nvme SSD as a startup drive and to host my containers, and 2,5" 5400rpm HDD’s for storage. Those disks are slow but get the job done, and despite running 24/7 there hasn’t been any failures.
I installed a passive cooling-block on the CPU and optimized the airflow in the case. It has 3 (small, as it’s a 1u server case) case fans that are controlled by fancontrol and only kick in when the cpu’s temp is > 60°C, which is not very oten. This also means it’s very quiet ;-).
Make sure to configure all power settings in the OS of your choice (powertop and tlp for linux).