From past experience, I can tell you if given the option I would recommend buying a car with the factory sunroof rather than an aftermarket. Too many horror stories about leaking, etc on the aftermarket. In many cases if you deal right the OEM sunroof won't be that much more than what you will pay for an aftermarket one.
My father was a car dealer for many years. You can certainly have a factory style sunroof installed, dealers do it all the time, and it will look and function just like the factory option unit. $1200-1500. But the above advice about avoiding the "bolt-on" old fashioned accessory type sunroofs ($200-300) is absolutely right - nothing but trouble and besides they look tacky.