You absolutely need sound deadening. The doors will need a speaker pack each and the boot will need a trunk kit. The more sound deadening you have in the boot floor, rear quarters and bootlid the more bass will come toward you and not out via a rattly numberplate and in to nothing.
I would suggest two speaker kits (£15 each), a wedge pack (£130 for the whole boot), dynaliner to help eliminate rattles, and a few spare sheets for inner skins etc. Look to spend a good couple hundred if going the Dynamat Xtreme route all in.
You could look at other manufacturers such as brownbread who are roughly the price and allegedly just as good
Previously: 2003 Peugeot 206 1.6 8v | 2006 E90 320d M-Sport, 19" BBS CH, Full Ice-cold JL audio install, August 2010 Total BMW 6 page feature car. | 2003 Nissan 350Z GT Coupe 286BHP
Now:2010 E92 LCI 335d M-Sport