Sorry for the late reply.
PhotographyLife is a good site. Their reviews are fair and reliable.
The optical difference between the 24-70/4 and 24-120/4 in their shared range is small enough that it should not be a factor in deciding. They both share the same construction. The 24-70/4 collapses for storage and therefore takes up less space in your bag. However, you have to remember to extend it for use. The 24-120/4 offers the extra range and an extra function button.
The prices for new examples are close, so you may as well go for the 24-120/4. On the other hand, as you discovered, the 24-70/4 is readily available used for a lot less.
Good luck!