Been converting friends and colleagues to refurbished Fujitsu Futros S720 for quite some time, because RasPis are not attainable. They go for like €21 with 2G mSata SSD and 2G RAM, it's dual-core AMD SoC with up to 1.65GHz CPU frequency and 4-12W power usage.

They can be upgraded up to 1T mSata (limit) + 1T (or bigger) 2.5" SSD in mirror, 8G DDR3 SO-DIMM RAM or for example w/ 2 port GigE Realtek Network card in the mPCIe slot (reusing m-sata plastic stando-ffs). The SoC (AMD Jaguar core (AMD GX-217GA)) even have AES-NI.

For the SSD, you just need L-Shaped SATA cable and USB header to SATA power cable for peanuts from ali express.

some more info at https://www.parkytowers.me.uk/thin/Futro/s720/ with photos of the motherboard.

S920 has even PCIe slot, so with riser or ribbon cable, you could hook up 4 port intel GigE network card and have 5 port firewall with pfSense, OPNsense or OpenBSD router.

I run mine with OpenWRT, because that mpcie 2-port GigE card is Realtek with lousy support in anything else than Linux.

What's uber-awesome with these boxes, they're passively cooled, so there's no noise apart some minor coil whine at boot.

EDIT: The box has mSata and mPCIe slots, but only plastic stand-offs in mSata. Ethernet card goes into mPCIe, reusing the stand-offs. Fixed that.