Aftermarket in-dash navigation on 05 prius

I am debating whether I should buy a used 2005 Prius, or a new 2011 model. One of the main concerns I have is money, which leads me to the older model. However, once I buy either prius, I want to install an aftermarket in-dash NAV system. After someone broke in our car to steal our external navigation system, my wife and I both want an in-dash system.

On the 2100 Prius, this is not a problem, since the MPG information is on a display separate from the navigation system, and I have seen many systems which integrate nicely into the dash.

But is this possible for an older model prus? I'm not really worried about getting the buttons on the wheel to work or whatever, I just want an aftermarket navigation system which can handle the MPG information like the onboard display does. Or will I need to purchase an OEM navigation system for $3500? If so I might as well buy the new car...

 

Oooh, I see now.I though the navigation had to replace the computer. I didn't even consider replacing the radio...