The 7800DXL is a modem & router so for ADSL/ADSL2+ it is all you need, but note it won't do vdsl. If/when FIbre comes my way, I hope to keep the Billion, and use it with a separate vdsl modem... which I think is possible.
Also, I see one of our other posts mentioned VPN. The 7800 DXL does not implement VPN though some other members (more expensive) of the 7800 family do, I believe.
Have you seen the recent thread on the '8800 which is being widely discounted? Worth checking in case it manages to tick any boxes that the 7800 doesn't.