I've not used Wunder Radio, but I use FStream and Tunin.FM.
FStream
iTunes Store
Tunin.FM
iTunes Store
Both work well over Edge and 3G, but are very dependant on reception. But they are both FREE, so worth having a play with before you fork out the £3.99 for Wunder Radio. Whether or not you like FStream or Tunin.FM, at least you can try the audio quality and connectivity in your area, as they are all taking the same data stream over the airwaves, and should therefore have pretty much the same output performance.
FStream requires you to add the URLs for the radio stations you listen to manually (once only as it stores them), which can be a pain. However it is quicker to load and get back listening than Tunin.FM.
Tunin.FM has a load of pre-defined stations, but takes longer to load.
As for pumping it through the car stereo, I use my iTrip Auto:
Griffin Technology: iTrip Auto - FM Transmitter and Car Charger for iPod & iPhone
It sends the sound to a FM channel which I tune the car radio to. Please note that not all iTrip's work with the iPhone. My previous one didn't, and although would send the FM to the radio, it didn't charge the iPhone, and you got the annoying 'clicking' that you get when you put a mobile near speakers. The iTrip Auto however works really well. I don't have a 3.5mm jack in my car so couldn't use that.
I listen to FStream and Tunin.FM through the car stereo, but reception depends on where I'm driving. The FM transmitter sends whatever sounds come out of the iPhone to the speakers, but haven't received any phone calls when using it, so can't tell you about that.
Hope that helps!