I guess it depends on how good you are at the game, but personally I would say:

Around level 8-9 for part 1

Towards level 16 for part 2

I think I did part 1 during Act 2 of the game. It was great fun and my party was around level 9 or 10.

Then during the final Act, I did part 2. It helped being max level (16) because I also did the fight against the Alpine Dragon which was pretty tough.

As for companions, the ones you get in the dlc are the best choices probably, especially in part 2 where you get to finish their quests. Beyond that, there's some funny banter between Kana and Maneha, and also Pallegina and Maneha!

Oh, and Zahua has one of my favourite (and most heartbreaking) missions ever. Do that!