I will create custom armor and vanity sprites for terraria

No. I only make the sprites. You can implement them yourself or ask other talented people to create a mod for you.

Mods are inherently more complex since you can do anything with them, which I can't be bothered to deal with. Resource packs are much easier to do, and I actually recommend making one yourself rather than ordering the gig extra.

I can retexture the sprite and change the name and tooltip of an item as they are data-driven. I can't do anything about the code. So, if you want a hat that shows the player's original hair, it has to replace one that already shows hair in-game, like the Red Hat.

It's not strictly required, but that would be nice. Do as you will with the sprites.

Hypothetically unlimited until I start the animation process. Afterward, you will get as many as the package says. Therefore, I'll be expecting active communication from you. However, please remember to be reasonable with the number of revisions since I am still a human.

Head variants are mostly used for class-specific helmet items. Variants for the legs, meanwhile, are often used to give female players slimmer legs. For example, Maid Shoes item. It is only one item in the inventory, yet it has multiple sprite sheets and armor IDs.

They are counted as separate pieces, even if they're "attached" (put directly on another sprite) to the body or leg sprites. This is because they require a separate sprite sheet and therefore more frames that need to be animated.

Technically no, but close enough, I guess. If you want a pet to go along with your vanity set, just let me know and we can discuss it.

Yes, luckily, town NPCs have similar sprite sheets to the player. As long as the animation isn't too complicated, I should be able to do it.