2012 Prius -How to add Sat Nav?

Just bought this. Model ZVW30R. It has a JBL Greenedge head unit, and a Sat Nav button that when you press it says "System not installed." I'd like to be able to use it with a Sat Nav system. My dealer said he couldn't source the Fujitsu unit that Toyota used to use. I rang Toyota Australia and they said they could source a unit but it'd be about $2-3,000!! I found someone on eBay selling a head unit for $850 plus $100 for rear camera (because he claims it needs a PAL camera), but he's in another state and I don't want to pay an auto-electrician to install this only to find that in some way it doesn't integrate with the Toyota systems. Does anyone know if you can just add-in a Sat Nav unit, or whether you have to rip out the JBL and start over? If it's all too complex and expensive then I'll just buy a Tom Tom, in which case it'd be good to use screen mirroring (if this is possible), but can't find any info on this.