problem with binning the head unit is that you will have very limited control over the volume, the amp will havfe a gain or level setting, which should be set at initial install and not tamperered with thereafter. say you plug your sly right in to the amp, you'll either have it really quiet or really distorted, just the nature of the beast! you would be far better getting a head unit with an aux input. hide this under the drivers seat. then get your old style stereomaphone and fit it in the dash, from it's out put, connect to the aux input, set the volume on old unit and leave it, use the new unit to adjust listening level. to listen to sly pod swap it into the aux input!
if you choose to run all the speakers via an amp then you will need at least a four channel amp. personally i would run full range speakers from the head unit and then, just put a sub on to an amp! get the best quality you can afford, as mentioned the cheaper head unit won't sound as good and a £300 sony but the louder you get the more noticable it will be!
i would also advise that where possible mount speakers in a fully sealed enclosure, a box with a bass port (a big hole) will be louder but the sound quality won't be as good, the speaker won't be as responsive and it won't last very long at loud levels.
lastly if you are going loud or long you will want to consider uprating your alternator!
in previous installs i've run jbl gear, in my oppinion it's the best and sounded great when i had 2.8kw in the boot!!

if you need any help just shout!