24-85 vs 18-105
In my view there is no reason to buy a 24-85 for DX cameras. You have better/cheaper options in the DX offerings. But to answer your questions anyway, here is how I see it:
1) Is the IQ of the 18-105 equal to the 24-85 in the shared range? Particularly wrt color, contrast, and clarity.
The IQ of the 18-105 is very good, you won't see significant difference.
2) Is the IQ of the 18-105 better than the 18-55 in the shared range, wrt the same things?
I guess it is sharper and overall behaviour is better (CA, vignetting, flare...etc.)
3) Is there a significant difference in the size & weight? It looks like the 24-85 is shorter and bigger around, but not by much. It's hard to tell without actually holding them though.
It is very hard to imagine a smaller and lighter lens than the 18-105. It is really tiny and light compared to any other zooms of similar range.