If I were you I would spend it on some half decent front components (16.5cm) for about £100.
I would then spend the rest of the money making baffles then sealing and insulating the doors with fibreglass and dynamat.
If you are a bit handy it's the best way to spend your cash IMO.
Just get a sub, take all bass/treble off the speakers with a low pass filter and crank it up!