Hello
I am using older Atom ITX boards as routers:
Intel D2500HN, Intel D2500CC, Asrock AD2550B-ITX
CPU is more than enough for 300 Mbps VPN. Never gone over 30% load.
Not using QoS.
using gigabit PCI network cards in addition – single, dual and Quad.
from ebay.
Do NOT use USB stick, it dies soon!
Use small HDD or SSD.