The manufacture and model should be entirely irrelevant, just make sure it has Ethernet connectivity.
http://www.freebsd.org/doc/en/books/handbook/pppoe.html
http://www.freebsd.org/doc/en/books/handbook/pppoa.html
Some DSL models double as NAT routers, so instead of a PPPoE/A client.. just configure DHCP on the proper interface (..preferably login to the router and see if you can enable PPP passthrough).
USB modems are often proprietary, avoid them.. and you should be fine.