Return that system
If I had just paid that kind of money for a system that has as many problems as you describe, I would be standing in line to return it! Why would you put up with any of these problems?
As for changing out the drive, that is easily done and you can use SuperDuper and WinClone to effectively clone what you have onto a larger drive... again why would you consider it if the system is not "right" to start with?
Neil