Would just like to put the Orange Pi out there: wielding 2 Ethernet ports, WiFi and other creature comforts for < $20 USD [1]. It's more than adequate for running as a Pi Hole [2] as well as having enough left over to do other interesting tasks. Would recommend from personal experience some form of cooling is you plan to really push it.

[1] http://linuxgizmos.com/tiny-hacker-board-features-dual-ether...

[2] https://pi-hole.net/