Order of Costume

First thing first: Every Costume stacks, so no matter what the order is, you will get everything in the end. This is only for the reference just in case you don't know what to buy
Second of all: This list is assume you are using V2 and Monoweapon. Since you need to do that to commit to that weapon and get it to V2
GreatSword
Helmet of Knights Bestia > Dragonbone sword = Dragonbone Helmet > Dragonbone Armor > Everything else
Bow
Solar Deity's Bow = Admiral's Bow = Bow of Eagle Feather Chief > Sun Deity Set > Everything else
Dagger
Soul Collector Set > Rogue Ninja's Double Mask > Everything else
Staff
Ancient Arc Mage's Staff = Staff of Onmyoji = Mask of Plague Doctor > Robe of Plague Doctor > Everything else
Credit:
Sword: Edenlight
Bow: Ellezard
Dagger: ImWhoops
Staff: Nate