You will not benefit from a Sata 3 ssd on a Sata 2 port, any more then using a Sata 2 ssd. Some people like to go with a Sata 3 ssd in case they buy newer hardware at some point and want to move the drive to a sata 3 port at that time.
I'm not certain on "Is it true the most important property of an ssd is it "MAX 4K write IOPS"? How much bandwidth required for MAX 4K write IOPS up to 85K?"
Really I wouldn't put much into either of those questions, go with a Samsung 830 drive (which is sata 3) and you will have a very reliable fast ssd.