Well i have an Astra but I can try to answer some of the questions, the subwoofer goes in the boot and well if the Amp don't go under the seat think about putting it in the boot somewhere I have mine attached to the back of my rear seat (may be an option)
Which wheels are you looking at, try a holden dealer, I got the ones top middle but in 16' on the astra
Colour screen??? Do you mean where the screen is now or to replace the stereo with a dvd screen??? If a replacement is what you mean go sus out some audio places and then hunt around online and maybe get it cheaper