The idea of the expansion came up after the heroes of Azeroth dealt with so much external threats like the Burning Legion. In their own words, "so when we talk about what's the biggest threat in this world, is it the titan or is it the dwarf paladin that put a hammer in that titan's head"? As such, the enemy (at least during the early parts of the scenario) will be the opposite faction, a "vast army of world-destroying forces".
To achieve reliably 200% success rate, you need 1 champion equipped with Vial of Ofuscation (counter stealth missions), and the others can equip the +10% increased success rate per troop (some champions, like Falstad/Rexxar already have this passive and will stack). You want to counter the cursed status (success rate wont go over 100%) but you can skip increased time and additional costs affixes since they dont really contribute to success rate. Overall I would say that you need 1 Vial, 3 10% per troop Horns and either one 10% increased chance or one reduced duration item (it counters first strike missions). Using this setup, you will raise dramaticly your success rate which in turn will give you more rewards, but you will burn War Resources since you want to send troops to raise the success rate and we dont counter the increased costs affix most of the time. Its hard to evaluate how many missions you need in order to make-up the costs of the champion equipment, but in the long term is going to be worth.
You should equip new quest rewards when their item level starts beating out your previous set of gear. If you are currently wearing Tier 21 gear, it is best to let the quest rewards significantly beat your existing item level before using them. With that said, you will unlock Azerite pieces early on, which are worth using over Tier gear in most circumstances.