It doesn't matter if you buy one with a 3 or a 6 Gb/s interface because a hard disk's transfer rate is less than 3 Gb/s. Buy the one that best meets your requirements, regardless of it's interface speed.
So, you're saying that there aren't any restrictions like the Mobo CPU support, when it comes to hard drives. I can buy any hard drive I want, even SATA III ones?