Easiest Faction for Campaign?

[Paltmull]

that said Sweboz have the worst economy,

My Sweboz experience (which is quite limited) shows otherwise. If you conquer the settlements around the baltic sea, you will get a pretty decent trade income. And when you've built mines in a few provinces your income will actually be quite good.

Edit: And don't forget that you can cheat a bit and use several generals in your army, which means elite infantry for a small upkeep cost.