in sound systems, ur music quality is only as good as your worst components quality. usually this means speakers. if you get a high end system for ur car but use ur stock speakers as well then u wont get the full sound quality of your high end system.
for head unit, i would suggest pioneer 5250sd. it has front rear and sub out plus other bunch of useful options. for speakers, keeping your future in mind and since i like to build a system slowly, get rear speakers 6x9. pioneer 6902 if you have budget, sony 6941 if you dont. stay away from kenwood 718 as they are only good to be run on their own in a system without a sub. both sony and pioneer can be run initially without an amp. your next update should be a 4 channel amp. jbl cs60.4 or kenwood 4 channel amp are good.
after that you should invest in a good pair of front component speakers. sony for lo budget, fusion for slightly higher and pioneer 1720 for still higher. the last step should be purchase of a woofer of your liking with a suitable amp
if you plan your system this way, each upgrade will give a boost in sound quality and you will feel the difference.