I bought and used a Nikon 28-85 mm for many years, then while cleaning it I noted there was a small dead spider inside. I thought about having it cleaned, then I decided to keep shooting. The lens was KIA (felt from height in Instanbul) many years later... I bought a 28-70 as a replacement and out of curiosity I took a picture identical to one I took with the 28-85. The spider made no difference in the comparison.
Anyway, an ugly lens gives exactly the same picture of a mint one, as long as the uglyness is in the outside. I bought an ugly Sigma 400 mm for 30$, found that the glass was OK, it was only heavily scratched in the barrel. Check the glass, it is the only thing that will make the difference.