As far as quality for cameras goes choose CCD over CMOS, & go for the largest sensor you can get - usually " CCD ( ie. not the other common ¼" size).

For reversing only a camera with 90deg + field of vision
For rear vision (whilst driving) 30 deg to 60 deg field of vision.

For a single camera 60 degrees field of vision gives a good compromise. It's good enough for reversing & offers quite good depth of field.
For dual cameras my choice would be for 120 degrees field of vision for reversing (which I currently have) & 45 degrees field of vision for rear vision whilst driving. (I've had 60 degrees & currently have 30 degrees - 30 degrees is very good but does tend to distort traffic when it is close behind making a vehicle look a bit fatter & squashed but I can pick them in the monitor from hundreds of metres away).

The narrower the field of vision the better the depth of field. ie greater depth of field enables you to see approaching traffic sooner & vice versa, so a 90 degree field of vision camera has less depth of field & approaching traffic suddenly appear behind you 'out of nowhere'. I had one like that once & had a few 'scares' when vehicles cam up behind me muh faster than I was going.

My 30 degree camera gives pretty much the same view as a conventional rear view mirror.

I prefer my rear vision/reversing monitor to be a mirror mounted type as this just feels more natural to me.

My preferred supplier is www.nassecurity.com.au/rear-view-cameras/ They sell both cameras, monitors & most usefully cable & cable adapters. I am happy using the cheaper mirror monitors together with a good camera so if you buy the monitor from ebay you can be sure that if you end up male to male or female to female connections that for a few dollars yu can get the necessary adpter cable to make it all work. Do stick to monitors with 4 pin connections though. There are still some sold with the old type RCA connectors (Red, yellow & white) - they work but the 4 pin connectors are better & easier. I'm not sure if they have 45 degree cameras though. I learned recently that 'Safety Dave' is selling these now.

Rear vision cameras for driving with are great, but must be used as an adjunct to good mirrors.

Apologies if you are not interested in 'Rear Vision' & only in 'Reversing' cameras. Many folk tend to use the terms interchageably & it can get confusing.

As WAWT says above, mounting in the dash can be a whole other kettle of fish in some modern cars especially if you have steering wheel controls to communicate with. 

Thanks heaps for the advice Cuppa. good to know that information