Give me some advice on buying auto video player gps navigation !

I want to buy a car video player gps navigation for my car and one of my friends recommend me this website http://www.cargpsvideo.com/volkswagen-navigation_c80,he said that the products there are good, and there are various models of car's GPS. I went to have a look, everything there is so cheap, so I'm worried about the quality of the products. Should I believe this? Is there someone once buy things there? Who can give me some advice? Are there any other good choices? Thanks a lot!