I have been looking at doing something similar, although it's a non-Bose system I'm replacing. It's worth bearing in mind, however, that the above system requires you to buy a seperate (Parrot) bluetooth module, which bumps the cost up considerably. As far as I'm aware there is no way to integrate it with the armrest BT system, although someone may prove me wrong
Steering wheel controls etc are covered by the thread stated, I've been looking at this:
http://www.sextonsdirect.co.uk/alpine/p~CDE-103BT.html?cat=0
Or the 104BTi (essentially the same unit in grey with the iPod lead included!), the advantage being that they both have proper iPod control, can be linked to the steering controls and have bluetooth actually built in (as opposed to being 'bluetooth ready').
They are single Din units and don't look quite as smart as the W203, but from reading around the sound quality is excellent and the phone system works pretty well too.
Hope that's of some help!