1. Search for BU-303 and Rikaline GPS receivers in this forum there is plenty of feedback on these 2 receivers. I have the BU-303 ~$60 on eBay.
2. As far as accuracy most civilian GPS receivers have about the same accuracy, there are some higher end ones (DGPS) that offer slightly better performance.
3. GPS works best if the receiver has a good "view" of the sky, but many here have had great success with mounting them inside and even under the dash. Mine even locks many times inside my house.
4. Spend a couple bucks and get decent software, most will say iGuidance or Destinator here. The free programs don't usually include turn by turn directions.
5. Some people have tapped into the OnStar's GPS reciever, I believe it works similar to a serial port, but you have to be able to get the NMEA data from it. It has been done though.