I Recently serviced my 08 335 with nav and they loaned me a 2011 535i with nav. Iam a nav guy and dont like driving cars without it. I dont think i would buy another BMW without the 2009+ nav system. It might blow all the same whistles as its predecessor but just the sharpness of the screen and how fast it reacts to your command is something i would purchase a newer model BMW for.
Ya maybe its a 5000k difference but look at it this way, your ultimately buying a younger car witch in-turn will bring back a better resale. So put a smile on your face and getta 09