You can join a raiding guild naked as a bard. Mana song doesn't require gear, mana regeneration is not affected by instrument modifier.
However, the
blue diamond/
diamond resist gear is the best purchases you can make outside of raid gear. Quest a
Falinkan instead, its arguably better, and MR is super easy to stack on a bard (we have two songs that stack for like +140MR with a vendor drum).
Also, horn is useless, its only useful for offensive songs. Percussion and String matter more, drum for +resist buffs and Occlusion -resist debuff, string for +HP regen.