My "C" drive seems to be dead so I will replace it with a new one (fast/stable would be better). The MB is older so it runs SATA 3. I understand there are bottlenecks with SATA 6 on older boards but is there any advantage at all in the faster interface for this situation? An SSD is not an option.
Unless you are getting an SSD (which you said you aren't) there is no way you will bottleneck a sata3 or a sata6. Just pick a seagate, samsung, or WD drive that you can afford. I prefer WD black drives, but I know there are a lot of F3 drive fans out there too.