I asked pretty much all the same questions as you a little while ago.
Brilliant head unit with full ipod control: Alpine IDA-X303
Before you spend money on speakers put sound deadening stuff in your doors - Dynamat xtreme - fairly pricey but compeditiors dont come close to the quality.
speakers: personal choice but I would go for Focal in the front possibley in the back if you really want.
Dont bother with 6x9's if you are going to put a sub in the back just get decent fronts.
Amp: try get something decent 2nd hand, dont go lower than about
300w RMS - dont get drawn in by the number on the front that says 10000watts!
RSM wattage is what you want to look for
sub - up to you but
dont buy Edge, Vibe, Fli, Sony, Fusion or anything you get from Halfords, go for Alpine, JL, go for quality rather than quantity - 1000w RMS sub for £100 is going to very loud but the quality and clarity is going to be rubbish.
Just had my stuff proffesionally installed by Huets and it sounds amazining.
I made the mistake of buying rubbish components first and just wasnt happy so ended up spending more money on quality stuff.
Hope this helps