I find that AF is not very reliable for camera scanning. With live view (Nikon USA calls it electronic rangefinder) and manual focusing, I get better results than with AF in most cases. Focus should be on film grain and not on dust or scratches.
Anyway, before you buy, check that the lens you’re considering goes to 1:1. Many “macro” lenses go to 1:2 only. You could also consider buying an APS-C camera. As far as my tests go, they work as well as FF cameras.