Hi, I think I remember seeing your project mentioned on GameDev.net some time ago. It's a pretty impressive concept that's going to need a fairly customised engine. I can't imagine it being implemented in something like Unreal or CryEngine - you're going to have to write a lot of code to manage a world of that size, so I guess you need something pretty flexible.
I've also used the Gamebryo engine, and while it is very flexible and customisable, it's also quite complicated and can become quite messy to use.
I'm using horde because:
* Most of the big commercial game engine frameworks are very expensive.
* With a little effort, Horde can deliver the same graphical quality.
* Combined with other Open Source projects, you can easily integrate Horde into a complete game engine framework of your own.
* It's clean and simple.
Anyway, I'd be interested to hear about which engine you decide to use, as the requirements for your project are quite ambitious, so if you decide it's feasible then you're going to have some killer technology involved
