What sort of gameplay are we talking here, sounds like an MMO of some sort to me in which case you'll need to take dedicated server hosting into account, for development a home PC accessed over LAN will probably suffice.
It's not going to be an mmo, but it will be an online only game (servers of around 50 players seems to be the number to aim for).
It's going to be a coop exploration/survival game, but other than that I don't really know how its going to play. I decided to get some procedural environments and engine basics set up first so i can decide what sort of gameplay feels best for what i get done.
Do you know of any engines that seem to be capable of this without me having to integrate all these different libraries? I have integrated some basic libraries into an XNA Engine i made before such as LIbNoise and C#SynthProject, but this is new territory for me.