Going to have to disagree that you need to buy a machine before building a machine. My first CNC will be the CNC that I am currently building. To get the machine up and running, I will probably be pretty close to $1500, so that is a very reasonable budget for building a machine. The machine will also be a much more stout machine than equivalent sized machines that cost 2-3 times more. Also, the added benefit of building the machine is each step of the process I learn more and more about how the CNC will run.
What got me thinking about building my machine was I saw a couple machines that someone had built at a local woodworking show, and looking at it, decided that it was a feasible project. After doing some research, I got in touch with someone I knew who had successfully built a machine. He mentored me through the design process. Also between the him and the forums, I have found avenues to get questions answered as I have proceeded through the build. There is a wealth of information out there for those who want to try building a machine.
To me the biggest drawback is the time commitment. This is minimally a year long project. But then, the journey is what makes the project fun!