Hi I picked up my 2008 sti 2 weeks ago, and it's amazing I cannot recomend them enough. I opted to keep the 17 wheels but I do plan to change 18's as they do look a bit small, as for handling I think it will make a slight difference.
Did not go for the sat nav but the stereo is light years ahead of my 54 plate wrx, I'd personly save the money and get a tom tom.
On to discount, I used to be a car salesman up to 2 years ago so have good experience of this. The dealer will probally tell you they can't discount, but if you show you will buy the car at the right price a deal can be had. But don't expect a big discount as they only have there dealer margin to work with and have been given very small alications of sti's for this year. The dealer I bought mine from has a allication of 11 and I bought the 9th.
The deal I got was 25995 for the car but they gave me 9500 for my 2004 54 plate wrx ppp 4 door which is a grand over book. So I was happy. ( this was before the recent price rise).
Just go for it you will not be disapointed and remember blue is the fastest colour