Oh, I get it... You want to _build_ your own ADSL modem, instead of using a prebuilt one (like the majority of other people - make that almost everyone)?
Sorry, I can't help you there... But enjoy building it
I would just set up a Linux box with something like IPCop, buy an ADSL modem, connect the two and then use that as my network's gateway. But, if you really want to do that, then... go ahead!