Hopefully this reply can offer some assistance as I'm in the same situation as you. I have a 2016 QX50 without NAV. I have found out online that swapping out NAV parts on non-NAV models is not as simple as it sounds, at least for me personally. I use a Bluetooth receiver that transmits my phones audio over FM radio which works surprisingly well in my QX50, not much audio quality is lost. There is also a usb port in the armrest that you can use to play music as well if you're ok with having your phone plugged in all the time.
In reference to aftermarket options, I have my eye on NiftyCity because they offer an aftermarket replacement but its made to look OEM. There's also a video on YouTube showing it installed in our model QX50 as well.
In reference to aftermarket options, I have my eye on NiftyCity because they offer an aftermarket replacement but its made to look OEM. There's also a video on YouTube showing it installed in our model QX50 as well.