Generic advice: for this amount of workload (~20 users) pretty much any office is a good donor. Must have-s: 64 bit CPU architecture (AMD64), VT-D setting in BIOS for virtualisation support, as many SATA ports as possible, gigabit Ethernet, at least 4 cores and at least 8GB of RAM. Storage disks: depends on the situation.
I had issues with the on-board Intel 82579LM NIC, tried to install the drivers from linux-firmware but didn't work so instead bought a very common and cheap Gigabit card (Realtek RTL8169) with PCI interface (no, not PCI Express, the old legacy PCI)