I don't own a Prius but I've been reading various forums.
My understanding is that view out the rear isn't the best. Now you could have parking sensors fitted but the Prius an annoying reversing beeper that can only be heard inside the car, so if you had reversing sensors those add to the beeps being heard inside the car, possibly making it difficult to hear which is just the reverse gear engaged and which is the parking sensors. Without owning the car, I can't tell you how acceptable the rear visibility is, if I were you and unsure I'd ask for another test drive,
When looking up post-codes on the GPS, you can not apparently enter the full post-code, but enter of form AA1 2 and then you look for the road that you are interested in - I don't know what the interface is like though.
I own a portable Garmin GPS which I use in my Honda Civic, and if I had enough money I'd have got a built-in GPS, just because I am so sick of installing and removing the stand/sucker from the windscreen. OTOH one advantage of a portable device, if it can easily fit in your pocket, is that you can mark where you parked the car, and then can easily find the car again if you forget where you've parked it.
I love gadgets, I'd buy the full tricked out T Spirit, but then that is easy to say that when it's not my own money.
Don't know if this has been any help