It helps to have different classes in your party, but once you understand the mechanics you can beat the game with many different combos. Picking a good crew can make the game much more enjoyable due to party interactions.
Dragon Age 2 was an ok game; not nearly as good as the DA:O with much more of a console feel to it. I don't even remember the story. Probably better to skip that one and play one of the many excellent RPGs from the last few years Haven't yet played Dragon Age Inquisition, but it is lying on my desk collecting dust.