Hi there, I agree that it is good to develop a basic one and observe to learn.
However, if you insist to first try to simulate, you can try Linear Tech. Sorry to say that I cant provide you the exact link to this software but I believe you should be able to google for it.
As for developing one to observe and learn the behaviour of the switch mode power supply circuitry, I suggest you to start with a basic boost DC/DC converter, which is much easier as compare to others.
Hope to hear from you regarding your progress...