This is a bit moot now, but
this machine only supports 3 gigabit SATA. Was there an issue with some SSDs dropping down to 1.5 gigabit SATA? If so, it probably still felt faster than the original spinning hard disk anyway, but depending on what you're doing (multimedia work, mainly, but also swap with low RAM situations) that additional performance is important.
The 850 isn't newer than the 750, the difference between them (they are both of the same "generation") is that the
750 is a lower ("budget") grade drive.
960 is the higher grade (PCIe NVME) drive, and is a generation newer than both the 850 and the 750.
They're all good SSDs, if the price is the same, I'd probably pick an 850 over a 750, but the benefits of an 850 over a 750 will almost certainly not show unless you have a more modern computer with a faster 6-gigabit SATA connection.
Ultimately, what I do with older machines I want SSDs for is go to my local office supply chain or big box electronics retailer and pick up whatever reasonable capacity I can find that's cheapest. I don't think (on a system this old) you'd notice the difference between any of these Samsung drives and a less expensive one from WD/Sandisk. I'm using a Sandisk 240 gig disk in my Mac mini (which has 6-gigabit SATA) and it's fine.