Is your current double din nav the factory one? Either way, if you have a double din that means that your a/c controls are where the sunglass compartment used to be. If you want to go back to single din, all you would have to do is buy the single din kit from metra/scosche/best kits/ or american international and then relocate your a/c controls underneath your single din, and reinstall the sunglass compartment where the a/c controls are now. They all sell the single din kit for about $12. If you want to use the hk factory amp, with an aftermarket deck, you will have to get the amp integration harness, it has rca's instead of speaker wires. Metra has that as well.
Once you go to a single din aftermarket setup, you have almost an infinite number of options. Yes you can find a nav ready single din. Usually they are flip out screens ( 7 inch) and cover your a/c vents, so most people don't opt for those. But like you said, nav isn't a huge must, so pick a deck that suits your needs and get to it.