I’ve been exploring netbooks and other devices with low-power intel atom processors, then attaching external usb drives for storage. With a low-resource OS, like ubuntu xfce, etc., the speed of these is pretty good, the power usage is minimal, and they have built in battery backup (laptop battery). Most of these are dual core, run 64 bit, and can have between 2 and 4 gb ram.
There are some thin clients with same specs (sans battery) in which an internal hard drive can be installed.
Not high end processing, but from all I’ve heard @dirvine say in interviews, etc., they should be more than adequite for nodes.