Linux has recently got native framework for control of switching chips called "Distributed Switch Architecture." This turns a lot of very cheap hardware with very basic hardware switching chips into high performance routers.
EDIT: No, actually, I was mistaken - there is one Broadcom series supported now I look again, BCM53xx, of which the one in the article does indeed appear to be. Looks like since the 4.8 kernel though, so not in the 4.4 kernel that comes with the device.
So going for a cheap, 2nd hand router supported by OpenWRT is probably the easiest and cheapest way there.
Their table of supported hardware for each of the platforms now using DSA probably includes all of the most affordable devices, since OpenWRT is mostly focused on consumer-grade equipment (and mostly routers/APs, but they support some purpose-built switches using Realtek CPUs).