OK, not the most exciting thing to do with kite flying, but for any traction activity a helmet is essential. It's better to break a cheap helmet than your skull!
What kind of helmet will depend on what actually you are doing - first any helmet is better than none, second some helmets are going to do a better job than others. After I got the kite buggy in January 2004 I simply used my normal cycle helmet (above right). I also know of other fliers who are quite happy using a cycle helmet all the time so I think they do the job. However after forgetting my helmet a couple of times I decided to get a dedicated buggy helmet - after a chat in my local shop I decided to go for a skate helmet (above left) - it's closer fitting than the cycle one and seems to give more protection to the back of the head and temples.
If you are going to be doing a lot of traction activities I would advise checking with a good shop to get something appropriate.
Depends on type, but around £20.00 for a skate style.
Kite Keepers (or Kite Killers as they are sometimes known) are a way of saving your traction kite (quad line only - this isn't appropriate for dual line traction kite) if (or when) you have to let go of the handles. They are a thick Velcro strap that goes around each wrist and lines that attach to the brake (or bottom) lines of the kite a few centimetres up. Letting go of the handles causes the brake lines to be under full tension with no tension on the power lines. This causes the kite to lose all power and - usually - flutter back to the ground.
The advantage is two fold - firstly they benefit the flier as the kite doesn't end up blowing down the field/beach, it becomes a trivial task to collect it and either fly again or put it away (otherwise it's a lost/damaged kite and/or very tangled kite lines). Also you know that if something goes wrong you can simply let go rather than holding on too long. Secondly it provides safety to other people on the field. Letting go of a handle when the kite is fully powered up will cause that handle to fly away from you very fast - which would cause injury if it hits anybody. Also a run away kite would have the potential of the trailing lines wrapping around other people and again causing injury.
There is some debate about the wisdom of using Kite Keepers in a buggy - I have to say from my experience there is no problem - you simply have to be aware of the kite keepers and keep them away from the buggy wheels. In fact you have more chance of having to let go ("ditch the kite") in a buggy than simply static flying when a fast turn goes wrong (trust me on this one!). So far I have ditched both the Raptor and the C-Quad in a variety of different wind conditions without any major problem to me or the kites (the C-Quad is more difficult to sort out afterwards than a Parafoil kite such as the Raptor - that is just one of those C-Quad things).
Kite flown on bars rather than handles also have similar safety systems - but as I have very limited experience of this I can't really comment (yet!).
Cheaper 'no brand' Kite Keepers cost around £10, Flexifoil branded around £26. Flexifoil (and maybe other manufactures) are now fitting Kite Keepers as standard on their Bullet and Blade ranges.