I used a Nikon Coolscan III before, and it worked good, but I think it was a little slow. Even though it used a SCSI interface.
But the Coolscan V uses USB 2.0, supports higher resolution and should be faster then the Coolscan III.
Either scanner will work with any PPC Mac, no Intel support
If you want to save some money, maybe buy a used Coolscan III, and a SCSI card for your G5.