Inexperienced rider breaking a horse?

I'm considering starting a horse project with getting a cheap horse and trying to transform it. I know this would be a time of learning for me, as I'm not very experienced in handling horses and riding them, for that matter. However, my uncle knows a lot about horses, and I'd be boarding so I'd always have someone that I could refer to and ask about horses. What would you say about doing this? Any suggestions of a specific breed I could try? I really don't have a lot of money so I'm trying to think of what could be affordable. I've been looking online at some of the inexpensive offers people have suggested for their horses. Please write as much as you can because I'd really appreciate this. I own a miniature horse and he is 4 years old. I don't see him much, however, but he's pretty much a companion (I used to do agility with him, but I haven't trained him in anything major like driving).