I think the most important element is longevity, and for that you need passive cooling that stays under 50-60 degrees Celsius at full blast. You also need disk solutions that are easy/cheap to replace and sturdy. That means old SD cards (SLC or MLC in that order) or old SSDs at ~65nm with 100.000 writes per bit. Modern NAND only has 10.000 WPB.

I still have my 45 nm Atom D510MO running 24/7 since 2010 in a passive heatpipe+sink case and 14 nm NUC + Atom 8-core in Streacom passive cases since 2014 and 2021 respectively. I'm waiting for the SSD in the NUC to fail. The recent Atom runs on X-25E that I know will outlast me.

N100 is 3x the performance per watt of the Raspberry 4, but it also costs 3x. I prefer the lower cost of the Raspberry 4 that allows more redundance even if that means more "garbage" has to be manufactured until that redundance is applied. Combined with cheap heatsinks and SD cards I already have.

The low power server of excellence is the Raspberry 2 at 2W TDP; I can have that running on solar only with a 100Ah battery bank... I think it would stay on a week without sun. The only 32-bit ARM multicore processor with vanilla linux that will ever exist for eternity, with the reduced complexity that brings!