Amplifiers – many of the units today come with built-in amplifiers. 
However, you have to take note that external amplifiers are far better than the pre-installed ones because they give you the opportunity to tweak the systems bass levels, midranges, and frequencies to give a balanced sound. However, if you prefer a separate amplifier, there is a need for you to use crossovers for the amps and the speakers.
Head Units – when building a car stereo system, you may either use the head unit provided by the factory or simply buy a new one.
Just take note that the ones, which come from the factories, have no pre-amp outputs, thus disallowing you to make use of external amplifiers. Replacement of head units, you need to consider your chassis size.
Installation – when it comes to head unit replacement, you need to know that this is actually a tricky thing to do. However, rest assured that you can do the trick simply and easily with the right kind of tools. Along with the right kind of tools, you also need to have a good electronics background and of course, patience.