Are you planning on trying to view the image on the vehicle's own display screen or get a complete system with a separate display? If the former, a vehicle-specific forum might be worth asking on. For me, the frame rate of the camera and the refresh rate of the monitor are critical - maybe more important than resolution, in fact. I had a Peugeot 5008 with a factory reversing camera on it, and the setup was rubbish. You could reverse into something, and about half a second later, watch yourself hitting it! (Not that I ever did, but you get my drift...) I think anything less than about 20 frames per second is going to look quite jerky and laggy.
Also worth getting reviews off the performance in low light. A numberplate mounted one will be closer to the reversing lights.
Lastly, if you can get one with a cover that flips up when you engage reverse, they're a million times better than the ones without covers, where you have to get out and wipe the lens after a run in the rain, before you can see much.