Re: Mascons....
Phil is right about Mascons, but their influence is felt at higher altitudes too (perhaps less).
From the Apollo era, I remember reading that 15 km was the lowest possible altitude the CM could reach (as lower, they'd risk hitting mountains).
An engine would be needed anyway to compensate for mascons.
The thing is: orbiting the Moon would mean a speed of roughly 5,000 km/h; maybe it would be tricky to get very sharp pictures at that speed, so low, but the higher we go, the more powerful the telescope on the camera.
Raoul