If you start with the head unit you will most likely blow the speakers, even if you don't want them any more. Like Tony says, start at the speakers. If you want quality, you'l need components which are capable of high(ish) levels and are able to reproduce moderate levels at the quality you require. Normally this means setting levels so that you won't be able to turn the volume up far enough to bust things.
Speakers -> amp -> head unit.