That’s great that you want to get into dirt biking. And it’s great you’re asking questions. It’s kind of hard to guide you from a forum , especially since you’re 13. Do you know anybody else locally to you that has a bike already that you could ask for some help? Have you ridden at all in the past? Go to a local race and just walk around and talk to people. There’s lots of people that will be willing to help. If you have any specific questions you can always ask here but it’s always good to have somebody you can trust locally.
Well you are a lot farther along than a lot of 13 year olds!! As far as I knew you saw a dirt bike in a video game and thought it would cool to have one! :-) I like the learn as you go. Nothing like experience to teach you about all kinds of things. Too many people are afraid to just dive into things because they might fail. Failures have taught me much more and any success I've ever had.
The YZ125 is a good solid bike and hasn't changed much for the past 10 years or so. As you probably already know, the price is very much based on the condition. The good thing is that 2 strokes are fairly easy to fix. A brand new one is $6500. There have been a lot of people buying up 2 strokes so that will drive the price up if you can even find one.
Closer to $200 if you need a piston. I usually swap out the piston once I’ve got the jug off.
The newer the bike the better. Just due to all the other parts are going to be wearing out. Besides the engine parts there’s all the bearings like linkage, wheel bearing, steering stem. They can run into some money too. If it’s an older one and the linkage is tight and the bearings don’t grind I would consider it. If it’s running that is. Another trick is to take a look at the air filter. If it hasn’t been serviced you probably want to avoid the bike altogether because that means nothing else has been serviced. That’s the easiest thing to keep clean.