Not to sound like a dick, but this is all coming from someone with 4 years experience and 4 different bikes, but a 125 dirt bike is no crotch rocket.
When I first started out, I was similar to you. Not a whole lot of experience but I wanted to learn to ride. So what I did was to save up about $1000, bought a old junked Suzuki GS500 and rebuilt it. It looked like ***, but it ran strong; it wouldnt hurt me (emotionally, not physically) too much if I downed it, since the bike was only worth $500. I beat the piss out of that thing. Learned everything I could about basic riding, traffic, road awareness...really the day to day stuff that you take for granted. While on it, I took a few rider improvment courses from the motorcycle safety foundation before I even considered getting a bike and dropping a serious amount of money on a new one.
If I were in your shoes, get a bike taht needs some work. Rebuild it with your uncle, have a fun time doing it, and ride the piss out of it and gain the experience you'll need for a real bike in the future. Dont start off on a bike thats too much for ya with the intention of "growing into it." Bullshit, your young, and again, not to sound like a dick, but young riders are hotheaded. You'll want to open it up, and when you do, and it dumps you, you'll regret it forever.
...just my $.02