Many different harness to choose from, some have a wider back rest offering more coverage and support, others are more flexible or stiffer. Price varies with each harness feature's, and stiffness. We recommend sizing down when choosing sizes, harness must be worn tight so there is minimal movement. Most kiters use waist harnesses because it gives you more mobility and control while doing tricks and riding waves. Seat harnesses are great for weak or sensitive backs and for learning.
Vest are a great way to protect your ribs from impact against the water on bad falls, they also offer you some flotation but keep in my mind this are not coast guard certified life vest. Life vest are available as well and marked as PFD's.
Helmets are also very important at least in the early stages of kiting. We really recommend foiling the first times with a helmet, they are very comfortable you might end up wearing them all the time. Better !!