Right, remember that the brembo's are standard on UK cars, so you not getting anything extra there! I'm assuming that this is a UK car?
Bearing in mind that the list price of a NEW GT with sat nav is over 31k then that a great deal if you can get for £25750, especially if its only done 400 miles AND with Sat nav!
Pads are dead easy to change, but you won't need any for thousands of miles yet, unless you want to specifically change to something more agressive. EBC Redstuffs are a popular choice and couple of traders on here (CS and Bigphil) do for about £99 all in!
Discs are a bit more varied in prices depending on what you want, but again you should not need any at this stage!
The sat nav is not linked to the bose system apart from obvious sound coming through the system. You should be able to change if you so desire to another system with the usual limitations. Look in the ICE & ELECTRONICS sub section.....
But you will be able to change the system and still have sat nav....
And you correct, 2006 cars had 296BHP (300PS)!