My ISP gives me a box that terminates the fiber and has ethernet on the other side. They also rent and sell routers that are configured to handle the pppoe and vlan settings needed for the WAN interface to this box. Plenty of routers can do this, and a dedicated Linux box like you are proposing should work, or you can throw a cheap managed switch in between if not. The hardest part is knowing what settings are needed (e.g. I had to call my ISP to ask for the pppoe password).
DSL standalone termination is still widely available, as are standalone DOCSIS cable modems.
