Companions are personal preference really. I'd take Kana over Hiravius because I like his personality and he interjects with interesting dialogue more than anyone else (at least it feels like he does). But some people find Hiravius hilarious. Likewise, I like Sagani over Grieving Mother, but they can fill the same role more or less. All of the party members can be very effective.
For your Paladin, go Weapon Focus: Soldier and you can switch between Greatsword, Pike, or Warhammer + Shield as you see fit. I'd be for going full-on two-handed, but there is a great Paladin-exclusive shield in a shop early in the game.
The best stats for dialogue are Resolve and Perception. Intellect comes up too, but it can often be replaced by Lore. Might is the most important for dialogue if you want to play an "Aggressive" or "Cruel" character.
Unless you care about achievements, I would start at Normal and switch to Hard when you have a full party of six.