Maybe Intel Xeon D servers. Something like the barebones Supermicro SYS-E300-8D. More expensive ones are SYS-E200-8D and 5028D-TN4T. These servers are compact, low power, not too loud, with lots of ethernet ports.

Lots of ethernet ports so you can put the masternodes in an isolated or DMZ network separate from management and other VMs for security purposes. Use a hypervisor like VMWare or Hyper V. Can run many masternodes in VMs because those servers can take 128 GB ECC RDIMM.

Entry-level Dell PowerEdge is probably fine if you wanna go cheaper.