You'll find armor sets very often, so that won't be an issue.
The Grass Crest is better on your back due to the stamina regen, though there are much worst shields to use. The Dragon Crest is the anti-fire shield, though it's outclassed by the Black Knight Shield. The latter is a drop item, though, so unless you feel like farming black knights, I suggest using what you have.
The Irithyll Straight Sword I don't know much about, other than the frost it does (which is situational) and that it requires twinkling titanite to upgrade.