Picking up a 2001 S4 this weekend...

Haven't heard many good things about the factory BOSE system, and looking to replace it as soon as possible. I don't have much experience with car audio, but attempted to put together a system that works well (to the best of my knowledge). Looking for any tips/suggestions regarding the audio components, or the car itself. Also, I realize an installation of something like this may be costly... can anybody ballpark a figure?

Headunit-
----Alpine CDE-9874
CD receiver with MP3/WMA/AAC playback

Amplifiers...
----Alpine MRP-F550
4-channel car amplifier
90 watts RMS x 4

----Alpine MRP-M850
Mono subwoofer amplifier
800 watts RMS x 1 at 2 ohms

Subwoofer...
----Alpine SWR-1542D
Type-R 15" subwoofer with dual 4-ohm voice coils

Speakers...
---- No idea? haha
Any suggestions that will fit in stock locations as well as work with the components listed?