Do agree re the gain between the Dell PowerEdge and SuperMicro etc. The PowerEdge is a C222 chipset with an Intel® Xeon® Processor E3-1240L v3 chip fitted
The small and quiet option I have is the Atom based firewall unit that has Intel 210 NICs.
The base chassis for the SuperMicro is circa £350, but will need RAM, CPU and SSD for that. I have CPU and SDDs already, but the RAM for the SuperMicro is DDR4 ECC so not the cheapest.
Atom unit is too slow for Sophos XG UI