You wont find a headunit with rms figures, they are always max power, my 2004 alpine headunit is 4x60w, so would be about 4x25-30wrms
For the best sound quality and sound staging, your not realy suposed to have any speakers in the back, just a small sub.
I'd go for a top of the range alpine headunit.
With regards to the speakers, you can get coaxils, these are simple speakers that have the tweeter built into the centre of the speaker and can be had pretty cheap, then you can get 2 way components, these have a seperate tweeter, which can be mounted anywhere within reason, in a 306 they would go on the A-pillas or where the interior wing mirror covers are, you can get them pretty cheap aswell, but you pay for what you get.
Genesis, focal, phoenix gold, hertz, rainbow are some of the top quality brands.
Dimma + Gt35 turbo =
Solid rear beam mounts £130