A few horses I've looked into haven't even been started hence they are very cheap. But on the flip side...No one has abused them, bullied them or taught them bad habits yet. SOoooo they are pretty much blank slates.
My question is, if you get a youngster who learns to trust you, is willing and has a pretty mellow attitude, how tough is it to saddle bride and start them? I've saddled my mustang mare when she was younger...but I WOULD NEVER bridle her and get on because there's something that just radiates from her that I don't trust.
I mean, would I just ruin a horse? I'm just curious what the opinions will be.
The old timers say: GREEN + GREEN = BLACK AND BLUE
That means a green horse that has little to no training (no training in this case) and a green rider (a rider that has little to no experience with training a young horse) will almost always result in the horse getting hurt, the rider getting hurt, or both.
From the way you have written your post, it sounds like you are very inexperienced when it comes to training a horse to ride. If that is the case, I strongly recommend you to NOT try to do it yourself. While I do believe a blank-slate horse is easier to work with than an already-started horse because you get to start from scratch and train the horse the right way, I also believe that it is very easy for someone like you to ruin that blank-slate horse and/or teach them very bad manners even though you don't mean to or don't intend to.
It is the lack of experience and lack of knowledge that will cause you to inadvertantly make mistakes when training this horse from the ground up. It is much more complicated that throwing on a saddle, climbing aboard, and hoping they decide not to buck. It takes a long time of ground training, teaching respect and discipline, sacking out, giving to pressure, and so many more things before you ever even think about getting into the saddle for the first time.
My advice to you would be to buy a horse that is older, experienced, and been-there done-that. You do not sound at all ready to buy a young inexperiences green horse.
However, if this is something you want to learn, start making contacts in your area. Find out who the good trainers are in your area, make an introduction, and ask them if you can tag along. Some may want you to pay, as you are technically going to be taking "lessons' from them in training. But some may let you follow along for free, for the sake of learning.
Regardless, it is best you see first-hand how a good trainer takes on a young horse, rather than you trying to black and blue bruise it through yourself.