I'm all for learning to do things on my own and wiuth only $350 invested, I would say that's not too bad to learn on. The parts themselves with a replate of the cylinder, piston, crank and bearings only run around $300-$400 for the KX100. However, having been through a 2004 KX100 complete engine rebuild, I will tell you that the Kawasaki's are probably one of the worst bikes for the home mechanic if you ever have to get into the bottom end. There is no way to properly insert a new crank into a KX100 without a hydraulic press and a special Kawasaki crank alignment tool. Others will tell you to heat the bearing and freeze the crank but trying this will only result in messing the crank up. So, without knowing what the bottom end looks like, the only way I would tackle this is to have a reputable shop do it.
If it were me, I'd still buy the bike for that money and part it out. You'll easily make double your money.
Good luck!