I got a single din Kenwood unit last summer and I've been enjoying it for the most part. I'd say make sure to get one that matches the interior color of your car. My interior is completely gray so it wasn't hard finding a black/gray unit.
The best place I found to look for a head unit is at
Crutchfield. They provide all the tools needed to rip out your old unit and fit in the new one.
I think to find a good unit that has both Bluetooth and navigation built in, you're probably looking at $500+. I bet you can get a cheaper one from one of the lesser brands, but apparently they have terrible build quality.