Hi Giordano,
It appears the Intel NUC is quite a popular option. Actually Beyers did develop a PBR option for One Nic appliances. We have been keeping it for Plus, but perhaps its time to set it free.
Alternatively you can run ESXi on the NUC and use a inside and outside vlan.
Kernel version 3.8.13. Most common USB adapters that are not bleeding edge should work. Biggest drawback I had with cheap USB Ethernet Adapters was that they limited throughput to 10 Mbps.