Welcome to the world of training horses. As for the horse,.... actually, I guess I’d want to know how you feel about handling him. I’ll talk, or type, about why, it will get a bit long though. It might help but.
Horses, and humans, like any social animal, have a very strong sense of sympathy. I don’t mean feeling sorry for something, what I’m talking about here is sympathy in its academic sense. That is, horses like humans, are able to sense or read how others of their species, and other species, are feeling very well; even to the point of apparent mind reading in some cases. So think of the times that you meet someone and you get a “gut feeling” that they are good or bad, that they might steal your car, or that you can trust them. There is actually plenty of scientific and sociological research about why this happens, and why we are usually right when we feel these things. We are a highly verbal animal, and we still have it innate in us, horses that are prey animal and mostly non verbal have this stuff in spades.
Now probably none of that is new to you, but what might be new is how emotions and feelings work in the human body (I would have to write a book to describe it, and it would be a poor book; if you want references I can give them to you). The point being that feelings and emotion are different and you can have them without even feeling them; importantly as you have them without feeling them you will somatises them, or your body will give off tell tail signs about how you are feeling, even if you don’t know you are feeling like that. Throughout the evolutionary survival of horses this meant that to avoid being eaten they had to be really good at picking up these kinds of feelings from their herd mates, and other prey animals they were sharing the tundra with.
So, what this means is that, if I understand correctly, it’s your first time to start a horse from scratch? If you are feeling a little unsure of yourself, or nervous, even if you try to hide it, even if you don’t actually notice that that’s how you feel, the horse will grab a hold of it straight away, and figure there’s something to be nervous about. This then leads to the chain of events I think we all went through at one time or another:
Horse: “what’s she worried about?” Rider: “this horse is getting worried about something, I’m worried!” Horse: “crap, she really is worried, what’s going on, your meant to keep me safe!!!” Rider: “Oh crap this horse is losing it, I’m more worried than ever!” Horse: “she’s scared of something! Screw this, I’m outta here”. Rider: “ouch that hurt” (brushes dust off and walks off to catch the horse).
The point is that you have to learn to control it all; you can't hide your feelings and emotions because a horse sees straight through that, you have to actually learn to shut them off and pull out others. I learned to do it slowly and it has helped me train horses. One of the best examples I have is a filly I have been working on. I was leading her past the shed where my uncle was working on his truck, as we went by he dropped a spanner and sent it clinking through the engine, followed by a few choice swear words. The filly crapped herself and nearly leaped on top of me and went to run past. Just as she was getting to the other side of the lead rope, the radio blasted out the Morse code beeps to signal the news, which sent her back over almost on top of me. Then back to the swearing and metallic sounding truck. She jumped back and forth about three times before she noticed I had stood there, still, without the slightest bit of fear (and believe me when you have a 2-1/2 to 3 yo horse nearly jumping on top of you it takes some doing to stop the fear; but, had I dived out of the way, it would have set her off worse). In the end, with all the noise from the radio echoing off the shed wall and my uncle swearing and climbing around the truck engine, she turned to face me and just put her head about 6 inches away from me with her ears about my chest height and relaxed; she knew if she was there with me she was in the safest place in the world.
When a horse gets that from their trainer/rider they get a leader and someone they can rely on. You can desensitise that horse as much as you want, you could dress it up like a sailing ship and it would be fine; but if it senses in you that there’s something to be scared of, that you are unsure and doubt yourself no amount of desensitisation will fix it. Don’t know if all that will be any help, but good luck with it all.
Oh yea, and yes, if you doubt for a minute that the owner can handle that horse it would be negligent of you not to impress that upon them, it may be the last thing they want to hear, and even be insulted, but I think you have an obligation to tell them.