If you want something cheap, you could go with Lenovo M720q's:

https://www.servethehome.com/lenovo-thinkcentre-m720q-tinymi...

They have a PCIe slot and can take 8th/9th gen intel cpus (6 core, etc). That PCIe slot should let you throw in a decent network card (eg 10GbE, 25GbE, etc).