In my car ive got Infinity Componenets up front and Infinity 6*9s in the back. Both running of the HU at the moment, although I did have an Alpine amp for the Infinitys but it was my brothers.
Anyway, the thing I hear most people doing is Buying a decent set of front speakers, amping them and then buying a sub for the boot. Because bass is no directional it will fill the whole car with sound and having the componenets up front will give YOU some nice mid range sounds.
Most people buy 6*9s and may later get a sub, this is a bad idea because 1) the 6*9s sit behind you so you cant hear them properly and 2) the sub is sharing the same boot space as the 6*9s and will basicly kill them.
So yeah have a look around at some stuff you like and spend your money wisley. Just because something says 1000W doesnt mean its going to sound uber good. Just slowly build up your system with decent stuff rather than £300 of crap.
But with your £300 id recommend buying a decent Sub/Amp package. You could get something like a 10" Amped Vibe sub for £200 and have £100 for compenets, but tbh I dont really know what subs are good and suck. But makes to stay away from would be Sony and anything that looks cheap. Try sticking to the likes of Alpine, Infinity, Phoienx Gold, Rockford Fostgate and Pioneer.
Anyways hope that helps.