Systems up to 500 watts usually aren't a problem but some people will add a capacitor to the system to ensure clean power for the subs. If you're going with a 1 KW+ system for that "can hear it a block away" sound then you'll need to consider a deep cycle battery (such as the Optima) or a second battery. You may want an upgraded battery regardless of power output if you listen to your system with the car off for any length of time.
I've had 500 watt systems in my last two vehicles and didn't do anything special for power--I just ran 4 gauge power off the battery to distribution blocks and 8 gauge to each amp.
I'm not sure if European cars are different, but in the States the battery is in the trunk on the passenger side--that makes wiring amps in the trunk relatively easy.