Minecraft wiki entry for Getting_to_the_worldborder, associated text: An efficient gunpowder farm. It can either be a creeper or a ghast farm. All it needs is it will supply all the gunpowder we need for the expedition. If you have a creeper farm but it doesn't give you many gunpowder, either expand it by adding more layers or make a new one from scratch.
A good sugarcane farm. Don't use those types of sugarcane farm that waits for the sugarcane to grow. Use an automatic and more powerful one.
A trading hall with zombie discounts. Zombie discounts are very great, as they can reduce trades up to 1 emerald only. The books needed are only Unbreaking III and Mending I. The other essential item you must get are XP bottles (to mend your elytra) which you can get from cleric villagers.
A bunch of elytras. In case your elytra breaks and you want to skip the XP bottle acquisition, then backup elytras are great.
An ender pearl stasis chamber. We don't want to travel a 3,750,000 block journey again so having a stasis chamber is very helpful to instantly teleport.
A powerful XP farm. Since you're gonna enchant a lot of elytras, you'd need a powerful XP farm to get XP fast.
Some shulker boxes. Shulker boxes are very helpful. You can either store here your backup elytras, your XP bottles, etc.
Totems in case your elytra breaks mid-air and you can't water bucket clutch in the nether.
Some food. Just in case you ran out of food.