My '07 does not have Nav, just the midrange unit, but I am also fixing to do a full audio upgrade. I plan to go with a Kenwood head unit, a 4-channel amp, new speakers, and a powered sub. Nothing window-rattling, just good clean sound. I may end up with a "multimedia" unit, which is one with no CD player, as my entire music collection is ripped. I will be adding a USB external drive with the music on it. I will also get Bluetooth for my Android phone, and a back-up camera. There are lots of choices for head units with these features/capabilities.
If you plan to do the install yourself, you don't have to be an engineer, but you will need to do some thorough research to know what is needed and how to install it. YouTube and Google have huge amounts of info on this. I've already spent a lot of time and will spend more before I get down to it. It's not a difficult process but it is quite involved. I suggest you go to the Crutchfield website first, add your vehicle, and read all you can there. The site has a wealth of information, they show you what you'll need to get and how to do it, and they can and do make custom harnesses to make it all plug and play. Some folks have said that their prices are high, but I think you kind of have to research that for yourself before you decide. I think that, while that may be true to some extent, it may also turn out better overall by saving headaches in completing the process. Of course, it all depends on how extensive an upgrade you want to do.
Anyway, I will be doing a full thread, with pictures and arrows, on my install when I do it in a couple of months. Good luck with whatever path you choose, and let us know how it turns out.