I think to retrofit your SR5 with Toyota OEM Nav you had better start out having the JBL sound system (non-nav but Bluetooth) -- option code EJ. That's because it is the same digital audio interface as the NAV version and uses the same steeringwheel controls and overhead microphone. Then there's the backup camera display the NAV does superceding the overhead monitor.
You do have plenty of time to research it though because the Toyota OEM Nav units are in short supply and will not be readily available for retrofit for months unless you come by a totaled '07 before that time. Ther is a Toyota NAV unit advertised out there that pops up every once in a while on the net but it is the older model than the new '07 part number.