Exalted Plains is a relatively big location that you can visit at any point of the game, after you gain access to Skyhold. It is optional to explore this location, because there are no storyline quests here. This does not mean that you should entirely ignore the Exalted Plains. You can receive many side quests here and obtain many valuable items.
The way to unlock this location: On the war map, select the "Scout the Exalted Plains" operation and complete it. It is possible only if you have, at least 8 Power points.
Recommended experience: Exalted Plains are not too demanding a location, when it comes to the recommended experience level. You can explore the majority of areas, even with a party at level 10-11. Only the fights against the stronger monsters require a party at level 12-13.
1) Exploring the major part of the Exalted Plains should not be to problematic, save the necessity to deal with numerous opponents. The only problems that you may have is with the bigger underground temple and in this case, see the walkthrough for the game.