Other than that I am looking at a networking middleware solution, like RakNet. It's inexpensive for the most part and is a nice old standard C++ library. Any other suggestions would be cool too:)
Perhaps to add the the outdoor terrain I will use Freeworld 3D and import the maps. I do believe Eric of C4 is going to add a vegataion update to his engine too, so we'll see.
And for physics, I believe there any many physX plugins and wrappers available for free.
So I think using the C4 engine and applying our own modifications, we can come up with some good multi-player goodness and eye-poppin graphics. Hehe
And of course, any suggestions are welcome!