lenses are usually named by their maximum aperture because it’s so important – for example, the Nikon 28mm f/2.8 has a 28mm focal length and a maximum aperture of f/2.8. Other lenses have maximum apertures of f/4 or f/5.6 (which don’t let in as much light), and some...
Moving to a different brand, e.g. Sony, is going to be more expensive because you will probably need to buy new lenses as well. A used R10 could probably fit, but not the R7. And he would need the adapter to use his lens(es), so count another 100 bucks. I do not recommend ...