To quote myself from another thread, "An SSD with SATA III interface will run perfectly well on an SATA II motherboard. You will lose some of the potential bandwidth, but it will be head and shoulders above an HDD. I know; I am running such a rig."
About the SATA PCIE adapters that you have seen: If they are PCIE x1 (one lane) then they are junk. Well, not really, but SATA III is faster than a single PCIE lane, so you will be choked by the PCIE connection even if there is only one drive on the controller. PCIE x4 seems to be the minimum.