I would recommend getting and good grooming kit, halter, lead rope, lunge line, fly mask, blanket, fly spray, shampoo, supplements/treats, and if you want a trailer. A trailer isn't needed at first but I would recommend it in case something happens. Other things like tack are better to get after you get your horse so you know what would fit well.
It really should just be essentials, like halter, lead and lunge line, fly mask/ blankets, fly spray, brushes, hoof pickand oil, until you get to know the horse. Tack and other things like that can be figured out after spotting your horses sensitive spots and what he/she rides comfortably in.
Tack, halter, blankets, etc.