Can you describe, as accurately as possible, the condition of the photos, type, physical condition, especially the surfaces of the photos. Isn't there any way to contact the photographer/studio that did your photos for a digital scan/conversion? Chances are, most of the work would have been shot on negatives, unless it was a big budget shoot.
I was once forced to shoot paintings using natural light because the artist refused to allow flash. Of course, control was a bit difficult. Have to wait for the exact right time of day/weather. The results, with proper filteration, was good enough to be used in an art catalog.
You can try it if you like. Look for an area where there is no direct light or strong colour casts like under a HDB block. Also avoid any rear areas where there is a bright or open so that there would be no glare or reflection.
With custom WB, it should be a breeze compared to what I had to go through. But if your prints are glossy, then it would be quite difficult. Slight uneveness in lighting can be easily corrected in post processing. You have to find a way to mount the prints as flat as possible.
Once you're confident you got it right, see if you can borrow a 50mm f/1.4 lens, or rent a macro lens. Yes, it will make a noticeable difference compared to your kit lens.