I bought a couple of $40-50 mountain bikes on Craigslist and pawn shops and then just started tearing them apart and overhauling them, one part at a time. I would watch videos, buy the tool(s) I needed and then ask questions here in the mechanics sub-forum. Some things are really easy and some take some practice. You'll probably need to invest about $100 in tools but it's very satisfying. I'm very unlikely to ever buy a complete bike because I have very specific tastes in what I want, component wise.