if you want as few devices as possible - set your modem in router mode... for example ...AVM fritzbox or Zyxel modems.. both support pptp over dsl as wel as PPPoA or equivilent sisco modemrouters can do it as well
my sbs connections all run on a cable connection with a budget adsl backup both are modemrouters with all port forwarded. so both WAN ports on my sbs server / gateway are configured statically with netid 192.168.x and 192.168.y
i had turned away from ppp bridges directly on the server, some time ago because i found it to be more easy to maintain to have a preconfigured modem/router stocked and installed rather than installing it on any server, especially since even with pppoe some providers like to push thair own 'drivers' on your gateway server
but i do agree, if you dont have sutch luxury or if you dont want it, than pptp wan should be availible on your zentyal server as well.