The starting broken sword gives an XP boost for every perk you unlock via equipment leveling. Wielding two of them stacks the XP boost. That's probably the most effective boost to grinding in terms of equipment. Tho you won't be able to fully unlock those bonuses until late game.
For early game equipment, armor is useful for boosting your effective health, tho you automatically unequip the armor as you grow the relevant body part. Upgrading armor increases how much you can grow before the armor auto unequips.
Honestly, companions have a much more significant impact on battle effectiveness than equipment does.