You are probably overthinking it.

A 8.5×11" sheet of paper, at 300dpi is (8.5×300)×(11×300)=8.415Mpix. But most documents I see scanned in professional life (contracts, etc...) and meant to be mostly viewed on screen) are scanned at 150DPI.

So definition-wise any decent camera will do, you problem being to find the proper lens and shooting distance to frame your documents.

Scaling down (reducing the size in pixels) should not significantly degrade the quality with current algorithms if done properly. Yes a 600DPI print can look better than a 300DPI one but you need to look very close, and this is true for printed text no for images. Of course if you pixel-peep the 300DPI one looks more grainy, but in real use you are not going to look it up that close. There is also the question of the output file size, and how much quality loss is acceptable.

At 10 images a day, you aren't going to have significant wear of anything (hinges of flatbed scanner, shutter of camera...).

Things to look for for both camera and overhead scanner:

• distortion: do rectangles remain rectangles? This is the quality of the optics. Note that flatbed scanners also distort, but in a different way.
• color aberrations if you scan color documents (and color is important to you).
• Depth of field: if you scan a book, the page will likely not be flat, so can you keep everything sufficiently in focus?

This said, if you need half an hour of set up before you can make you ten images (take out tripod and camera, set up, add lighting, make a few test shots), you will not do it in the long run. The best solution is the one which is practical.