Well... I had a half hour invested into a reply, and then my power cut out... Grr. So I apologize if I miss some points in this one.
I think that people would need to understand just how much time, money, and resources go into young horses/breeding. And how much is circumstantial. You can prepare, prepare, prepare, and things may still go wrong. It just happens. You have to know that if you breed, there is the possibility of the mare and/or foal dying.
I am glad that you are taking the time to ask. It seems that you want to be prepared and well-informed before attempting to do either of your above named things, and for that, I applaud you. It just frosts me, to say the least, when people are... backyard breeders or uninformed and then they get on here and complain, or ask for advice too late. Thank you for being a responsible horse owner.
That being said, make absolutely certain you are financially and otherwise prepared before breeding or buying a young horse. People often think they are ready, but realize too late that they're not. That will hurt both you and your horse. Not advisable.
I don't know the exact costs of breeding. There are definitely other members on here who can outline that for you. I know at one point, in some post, it was outlined, I'll see if I can find it...
As for buying a young horse, there's the inital cost. Then, there's your time. People underestimate the time that young horses need. Most of them, anyway, I have had a few exceptions, but generally, that's the rule. You can't expect them to be anything like any horse you've ever handled before, because frankly, they're not. And they don't know people and how they act, usually. So not only are you training them, you're teaching them about people, and their impressions of you will last their lifetime. Horses have memories that would put an elephant to shame. You have to be calm, but yet assertive enough that they won't threaten you. It's a tough balance, and it helps to have someone who can help you out with your first young horse. Young horses need consistency. Once you do something one way, that's how you have to do it. Until they get it completely, you can't really change it. That is so confusing to them. You can get away with that on an older horse, occasionally, but a young one won't forget. That's impressed upon them because it's the first time they've been asked. It's easier to create a habit than to fix one, so create good ones. Young horses will push your limits. It's always helped me to have them in a herd environment, just because the herd teaches lessons that you would have had to otherwise.
You have to think of what you want this horse to be. Will you really be able to keep it forever? What if you lose your job?, or if you go off to college? Sometimes things happen, and you have to be prepared for that. If you had to sell this horse, would you be able to? Bloodlines, performance records, temperment... these are a few of the things that should be considered in the sire and dam. You want a proven horse. The market is bad enough right now that you better put considerable thought into those. What kind of horse are you looking for? Eventing, reining, jumping, whatever it is, you want to get the best that you can. Also, if your horse would be registered, the breed registeries charge an arm and a leg these days. Every little change that you want to make, they'll make you pay for it. Vaccinations, vets, and anything that might go wrong... You'll be paying for those.
Young horses are EXTREMELY accident prone. Seriously. If they can hurt themselves, they will. They're like small children. Haha. You also have to have the facilities. Can you put your mare and foal indoors if the weather gets bad? Can you watch them on a camera, or will you be running out to the barn to check them every 10 mins? There's just a lot to it. And I'm sure there's things I haven't even considered.
And, sometimes things just go wrong. For example. I bought a pretty little coming 3 year old. Sweetest filly, but never been handled. At all. 5 months of MY time and money into her, as well as my trainer's, she goes lame in her front feet. The kind of lameness she might never recover from. X-rays, expensive special feed, 4 months of recovery time (NO RIDING or training at all) and special shoes later, she's good. But now, every time I shoe her, she has to have those shoes, and unless she makes a miraculous recovery, I'll have to pay for these supplements for probably the rest of her life. Not cheap. Plus, I have to watch her diet, not too much grass since she's prone to laminitis, there's some surfaces I can't ride her on, and there's a limit to the training/showing she can take, and there's more. And this horse is now 4. But that's just the risk you take.
I know I've missed some, my train of thought was lost with my first post, but other members will help me out. And sorry it was so random... Ugh, I just hate it when I lose a good post like that! Messes me all up. I hope I helped... Sorry if it was confusing in such random order like that.
To rein a horse is not only to guide him,
but also to control his every movement.