Assuming I can use my character & inventory from the main game? And if so, do I need the main game downloaded or just my save game files?
It's seamless, so you don't import your character from the base game to Expedition. Expedition is just part of UnderRail, and you play the new content more or less when you want (though almost all of the new content - like almost all of the base game - requires you get out of SGS first). And yeah, as Styg mentioned in a DevLog somewhere, you can use your existing character as long as you haven't actually triggered the endgame sequence yet, so you could even do the content after returning from DC.