Pullys make noises and a few seals leak.

They gotta stay profitable somehow, and it looks like you're it. Actually that's not fair on my part because I have no idea what "a few seals leaking" means. They might be giving you a real good deal. You're not tied to the GM dealer yet like you would be with a newer vehicle so consider getting a second opinion from an independent shop.

You didn't say why a new belt was installed or whether the noise was there with the old one. You didn't ask any questions either or for help so my only warning is to not use any type of belt dressing ever on a serpentine belt. If you do, that stuff will hold in small rocks and dirt and the squeaking will get worse. Even when all the pulleys are replaced as necessary, the only fix for belt dressing is to scrub all the pulleys with soap and water, then install another new belt that is not contaminated.