It all depends on what you want out of the system!
Do you want to use your IPOD with it?
Do you want Bluetooth for using it as a handsfree?
Do you want navigation?
Do you want it to play DVD and DVD recordable discs? (this means you can have 4.7gig of audio music on one disc!)
Do you want single or a double DIN unit?
Do you want a touchscreen?
etc
etc
Also are you sticking with the stock speakers?
If you are, then there is no point in going for a high quality head unit as the OEM speakers will not be able to do the sound output justice.
Lots of parameters to know