I'm holding off buying the DLC as I think it is still overpriced for what it is... I'm hoping that it will all drop when part II arrives!This game is very good, it's worth buying all the dlc too.
I have completed the below campaigns on very hard.
Chaos (also done on legendary)
Empire
Beastmen
Green skins.
I currently doing a dwarfs campaign on very hard and it's by far the easiest. Their economy is so strong compared to other factions that you easily afford 5/6 stacks of good troops.
If you play on higher difficulties the essential skill is the red tree one what that buffs your basic troops to plus 12att and def, some factions it does charge instead of defence.
Grombidal for dwarfs has insane campaign buffs in blue tree that benefits entire faction.
I found the Empire the hardest campaign, due to being in middle and possible getting into a war with everyone. Plus bearing the brunt of the even more powerful chaos invasion and Varg and skalling constant raids.
You can also play on very hard campaign and normal battle difficulty if you like. Just start a campaign on very hard, go into options and adjust the battle difficulty to normal. The Ai gets insane buffs on higher difficulties which can ruin immersion such as goblins spears wiping out drawf warriors with ease.