I am very much a list person, so I bet you know where this is going.
Make a list of things you need this horse to already know.
Do you want him green broke, or are you comfortable starting him? Or do you have someone to start him for you?
What do you consider green broke? WTC? Knows leads? Rides down the road or on the trail, or just arena ridden?
Do you want him already neck reining, or are you comfortable teaching this, as well?
If you are going for a green horse, ride him (or her, it's just easier for me to use one pronoun and I'm a lazy typer). You are looking for a horse that wants to work for you. It's okay if he doesn't know what you're asking, as long as he's trying to figure it out. If he gets frustrated easily, I'd pass. If he's not broke, you want to get an idea of how he will respond when you ask him for something.
I'd look for at least decent conformation, especially the legs and feet. Look for a horse with good quality feet, too.
If the horse is not built to do what you want to train him to do, pass on him regardless of how well you like his personality.
It surely helps to make a list (told you I was a list person
) of questions to ask over the phone. Things I typically want to know:
How has the horse been handled?
How does he respond to new situations?
A brief medical history, including vaccination status.
Has he ever foundered, tied up, had an abscess?
There are many more questions you could ask, but on a telephone interview, you want to keep the list fairly short. I'm sure other people will chime in with great questions as well.
I've had good luck buying unbroke horses and sending them off for a month to get started. Some people can handle a horse with less time at a trainer's, some people need more. There's no shame in either situation, just do what's right for you.