This is a dedicated ranged deck intended to pair with a high-threat Three Rings deck. The other deck will get the enemies, this deck will kill enemies with ranged. It should be good enough that Brand son of Bain uses his ability to ready heroes or allies for a special ability that requires exhaustion (like Elrond using Vilya or Galadriel using her special ability). Dori will help defense across the table, either doing it himself if heroes aren't available, or bolstering a high-defense character on the other side. Elven Mail is specifically included for Elrond, he will be the preferential defender if he has Vilya, since either he defends and Brand readies him, or he's ready for Vilya. But early on Elrond will likely be questing, so if he lacks Light of Valinor Dori may have to defend on his own.
This doesn't really have an absolute key card to look for when mulliganing. A weapon can be good to enable Foe-hammer, Elven Mail is great, though it may not be immediately useful, and on a combat-light quest one of the five 2 allies should be the target. On a quest with direct damage, Honour Guard may be the most useful card here. If you get a hand with Support of the Eagles and one or more eagles, it's worth keeping.
Gather Information may seem a bit odd in a deck that lacks must-have cards, but the Three Rings deck is just teeming with must-have cards, and so the side quest is largely for their benefit. The only sideboard card listed is Arod for location control (and because of a thematic connection to Legolas).