Would something like this suffice? -- http://pcpartpicker.com/p/3QcUs

I went with the low-end AMD AM1 platform because it's designed for low-power, dual-core processing, which would be sufficient for handling both the blockchain operations, and routing should you want that feature. I also included an extra gigabit network card since if you're doing any sort of routing, you'll likely want full-duplex functionality. With this sort of configuration, you can have the box directly connected to the network, running openWRT, and have all network traffic pass through the second network adapter - connecting the second nic to a gigabit switch would give you however many ports you would need for your network.