You do not need to buy everything at once. Some things the first day will not work without; others are easier to choose once the kitten is home and you know its habits.
Needed by arrival
A carrier. Not a bag and not your arms: in a car and at the clinic the kitten must be in a closed carrier. A hard one with a top that opens is more convenient — you can lift the animal out rather than dragging a frightened cat through a small door.
A litter tray and litter. A tray with low sides, and the same litter the breeder used. Change it later, gradually.
Bowls. Separate ones for water and food, and not side by side: cats dislike water that smells of food. Ceramic or metal — plastic holds odours and scratches.
Food. The one the kitten was eating before you. Changing food in the first days almost guarantees an upset stomach.
A place to sleep. A bed or a covered house. A cat needs somewhere it cannot be seen — that is a basic need, not a whim.
A scratching post. Put it up straight away, not after the sofa has suffered. Tall and stable, so the cat can stretch to full height.
Can wait
Toys. A couple is enough to start — you will not guess what it likes anyway. A wand toy works for almost everyone.
A brush. Needed in a week or two, once the kitten is used to being handled.
A circulating water fountain. Cats drink little and often prefer moving water to still. A useful thing, but not on day one: let it get used to an ordinary bowl first.
What not to buy
A flea collar just in case — parasite treatment is chosen by weight and age, not bought at random. Vitamins without a prescription. A harness for a kitten that has not yet settled into the flat.