I have a cheap version fitted to my Jowett van which has very poor rear visibility. It is kind of ok, the picture auto adjusts to the light so if the sun is behind you you don't see a lot.
Installation is easy, and to operate you just turn it on, I have it connected to the ignition so when I start the car it is on.
The one I have cost $35.00, needs to be wired in and has no GPS, just a camera.
I would recommend shop around $300 sounds a bit much, but then you get what you pay for.
Regards Barry