Mods can be hosted anywhere. I believe the intent is to set up a section for D:OS on Nexus, etc, but the mods themselves are just a set of files in one of the install folders, so it doesn't matter how they are distributed.
As Elwyn said, the game itself is DRM free, so for the Steam version you would only need the client to install and update the game (after that you could back it up, etc), or if you wanted to use Steam specific features (like the Steam Workshop).
Pre-orders could possibly come with (Steam) beta access (except from GOG) and ordering from the Larian Vault should give you a DRM-free installer, as well as a Steam key. That's how it worked for Dragon Commander, anyway.
