This document is a list of hardware that has been used for development and home use as a ROCK sensor. The first option would be to repurpose a machine if possible (minimum specs below). If your budget permits you can build a powerful system in a small form factor for under $1000.
Some of the more important aspects to look for are IPMI for baremetal remote management, dual Intel NICs, quiet, and relatively low-power. You don't want to have to wear hearing protection while sitting next to your infrastructure.