Well, the sky's the limit, really.
Primarily, I wouldn't recommend getting a unit with Navigation built in. As nice and "all-in-one" as that is, the navigation units are almost twice the price of an identical unit without navigation. Plus the maps are generally never updated, or you have to pay through the nose to get an update (if at all). In this case, I would very much recommend just using your phone, I'm going to assume you have an iPhone, or some sort of Android phone; anything that supports Navigation (Google Navigation, etc.) will be much
better than anything the headunits have.
With that out of the way, it comes down to what sort of unit you want. Are you happy with a Single-DIN unit, or prefer Double-DIN? Here's a picture of my old unit and my new one, in my KH. Personally, I think the Double-DIN is a billion times better, not only because it has more features and a touch screen, but it just "flows" with the dash and overall look so much better. More-so because I don't have an ugly pocket for random ****. Double-DINs are generally more expensive, but again, I think worth it. Because you mentioned navigation, I'm going to assume you already know you want a Double-DIN.
Next step is then, what brand you want. As you can see, I like Pioneer. Generally speaking, Pioneer or Alpine are the best units to get. There are cheapo no-name brands on Ebay, but I really wouldn't recommend it, as not only will the unit (in most cases) be more easily breakable, but you will actually get worse sound from it.
I've got myself Pioneer's latest model, the AVH P8400BH (or P8450BT here). I imported mine from the states as it was over $400 cheaper. Installing just the headunit is really easy to do yourself if you want to, if not, headunit installs are generally $50-$100.
Once the headunit is done, you need some better speakers to get that premium sound out of. It gets even harder here as there are so many more price-points, brands and "good" options. Again, in the "main" brands, Pioneer and Alpine have the best speakers. We can discuss the alleged
best brand "Boston Accoustic", but they're pretty expensive. I have Alpine S-Series 6" Splits in my front doors and Pioneer 6x9" in the parcel shelf.
Going past the spekears, I would recommend a subwoofer. Not for da fully sick doof doof
, but some low-end frequencies. Despite what anyone wants to say/judge about a subwoofer, it provides a much better low note (obviously). Doesn't matter whether you're listening to techno, or orchestral music. A subwoofer will give you the extra "oomph!" to really make a song sound epic.
As long as you're not blasting doof doof with maximum volume, I'd recommend just getting the 12" Kicker Kick-Pack. Has a decent AMP+Sub combo ready to go.
...I think that's everything, let me know if you want more info.