Maybe check some options with Intel N300/305. To be honest I don’t know how good Intel Quicksync is, so you might need an external GPU as well. You also need a power efficient DC PSU at low wattage.
It is also important what components you put inside, like low-end SSDs have the lowest power consumption and maybe are your best bet. I would also recommend you buying a 5400rpm power efficient HDD if you need it. Disabling all unnecessary ports in the bios can also help lower your power consumption and tweaking the power limits of the C-states, and enabling C10.
My suggestion is first try to play a bit with your current server BIOS settings and the power optimization in Linux. If your power consumption at full load is like 90-100, maybe consider swapping your current PSU with a DC one, as they have superior power efficiency and after each change evaluate the performance and the power consumption both on idle and on 100%.