well, in my experience, I don't think you'll need to go too far in a car the size of a sud to be too loud!
the ICE we put in my lads Polo comes to around 2kw total amp rating, and is spread around the car, with a lot of emphasis on the boot and bass end of things for his dance/dub tastes, and its ridiculous at nowhere near high volume on the head unit!
Rock music would actually sound even louder because dance music has a lot of voids in the sound spectrum, particularly the middle where the human ear is more receptive anyway, so a loud system for dance music, with all its highs and lows but lack of mid, would be blistering with rock played through it!
and we haven't had to use a high capacity battery (45ah!) or uprate the alternator, yet!
if he wants to add more, a bigger battery and power caps would be the first thing on the list, but unless you're building a sound off competitor, I don't think you'll need to do much at all really Harvey.
my own thoughts would be to try and house a nice larger battery than the stock ever ready, and see how you go, but I suspect you'll find it sufficient.
OOh, and on a different note, your flywheel, crank, pressure plate, rods and pistons are finished!
nicely balanced to perfection!!!! awesome!!!!