You might have a look on ebay, dell has been selling 755 optiplex desktops for less then 100 bucks.
Not sure how many PCI/PCIE slots they have in them though.. probably at least 2, plus onboard.
they run core2 775 socket cpu's which would likely be plenty of power.
The optiplex systems come in a number of sizes, the small form factor ones are very small.
otherwise you may look into a Atom ITX motherboard, they usually have at least 1 Ethernet, sometimes dual, and usually have 1 pci/e.
either option would be cheaper then trying to adapt a netbook.
If you need a SSD you could always get one of those or run IPCop from USB flash drive, or get a compact flash -> ide/sata adapter.
CPU power wise any new system would probably fit your needs, My IPcop runs on a 14 year old 400mhz celeron with 128mb ram and it runs like a champ.
Besides, there's not a laptop/netbook in the world that was designed to run 24/7/365.
Once seen a stack of 4 panasonic tough books in a cluster.
Seemed like a terrible waste of money to me but on the other hand it took up very little room.