What head stall would you recommend to teach a horse to lead?
Should it be leather or nylon webbing?
Should it have a normal buckle or a sliding buckle?
What are string halters like? Are they safe?
Nadine, it doesn't really matter what the material is, as long as the halter has a good fit and secure buckles.
I personally do not like the knotty rope halters, as they can inflict pain - some people use them as retraining gadgets (BECAUSE they inflict pain) but to teach a horse to lead, I would start with a few exercises to teach your horse to acknowledge you and respond to basic yielding requests. (halt, go, turn move over, back-up) with a reward to anything the horse is offering as a desired response, so there is a positive learning experience and not a response to fear (as in whip or jab first, ask later...!).
Also, I would start leading quite early before the horse can find out that it is in fact much stronger than we are.....
Tell us more about your "halter candidate" - how old is he and how much has he been handled so far?