I cannot comment extensively on the paint job. It sounds rather expensive to me. Maybe this sounds stupid, but why don't you just "gray" everything out? I would think it would be much easier to just match the tails, grille, bowties, and rims to the color that the vehicle already is. I understand if you want that "blacked out" look, and if your set on it - go for it, but it just sounds so expensive to me. To get a really good paint job, one that will last, and one that paints all the parts on the vehicle (inside the doors included) you are looking at spending a pretty penny. The other option could be to go "two tone." Have the lower half painted black or vice versa and leave the other half the original color. I really don't know. Maybe someone that knows more about having a vehicle painted would be able to offer a more educated thought on the matter. Anyhow, that's just my 2 cents on that subject.
As for the navigation question - I currently have the OEM navigation in my Tahoe. It is just....okay. What I liked about it was the fact that it was nearly all "plug and play", required very little programming and additional wires, looks good (meaning fit and finish of the dash), fits nicely, and works with Onstar (which I use). However, the navigation, to put it lightly, sucks. I mean, it's okay for what it's worth, but there are WAY better options out there. A head unit with Garmin or Tom Tom navigation built in would be much better off in the long run. On top of that, the interface is rather lacking. It has a pretty simple look, and that's about as far as it goes. If you want good navigation and a better looking interface, I would look somewhere else!