Toyota Nav

Hi, I am looking in to buy a first Gen Auris Hybrid (around 2010/11), and have been looking at the T-Spirit spec. There looks to be a £1000+ difference in price between cars with and without Toyota factory Sat Nav. 

I guess I have 2 questions; 

1. Is the Nav / Entertainment system any good?
2. Is it possible to retro-fit the Nav system at a later date (if so what components would I need)? 

Many thanks

Nick