The 3805 should be fine. The 3805 should be fully able to drive a full 6.1 system plus a couple of outdoor speakers on another zone. That's what it's made for. After that, well you might need another amp if you add more speakers.
Adding an external amp may give you better sound, it may not. All of these things are really dependent on what speakers you will have and how many. Regardless, the 3805 is a great starting point. You can always add an amp to it later if you choose.
Shinerman