I've always known about the Tara-Deva trade, and I know it brings in a nice income for Britain. Check back in the Britannia thread, I've mentioned it a few times there.

I think Germania are the "best" barbarian faction, and one of the easiest if you use the blitz at the start. They have the Spear Warband that can beat almost, if not all, first level units. They have great infantry later on, and Gothic Cavalry to sweep up. They're also one of the most unique factions, as they have a fair few unique units (Beserkers, Chosen Axemen, Spear Warband etc.)