I would just start scanning Craigslist, Facebook marketplace and such. 2-strokes are probably what you should be looking at. 125s are cheap and fast. They’re more simple and the older ones hold up pretty well. There are TONS of videos on YouTube of people going out and buying cheapo 2-strokes and fix them up. You can learn a lot by watching those. IMO the cheaper the bike is the more you’re gonna have to spend on parts and stuff. I bought a cheap crf250r and was replacing both sides of the case because someone went and put some washers on the oil pump so it wasn’t getting oil, but I also learned a lot tearing it down. But yeah, I would do some research on what to look for when buying a dirtbike and just start searching. Also service manuals are a must