Thought you were dealing with a new build. Looks like you might be wanting to move Windows to this other new build. This can be done but not really allowed according to the EULA of Windows.
The easiest way to find the OS drive is to look for Windows folder on it, additionally its usually the C: drive letter. Looking at the drive size is one way of knowing which physical drive is the one with the OS on it such as if you have a 250GB HDD and a 500GB HDD and when looking at My Computer at the drives it shows C: drive as 126GB free of 238GB then thats the 250GB HDD. So when opening the computer and looking in you then can see the label on the drive and know that is the drive with Windows on it.
Samsung makes good SSD's and that price seems good for the drive.
If your thinking about installing a SSD and keeping one or both of the HDDs inside the desktop computer, then I would disconnect both or any hard drive that will remain and have only the SSD connected. Install the OS fresh to that SSD. Then with that SSD connected to SATA 0 or SATA 1 connect the other drives to SATA 2, 3 and so on. This should make the SSD the drive that Windows boots from. And if keeping the hard drive installed with the prior Windows build on it you could have an alternate boot option to skip the SSD and boot off of the old build of Windows if ever needed. Sometimes you need to go into the BIOS and select what Hard Drives are in order of boot. Such as I have a Biostar motherboard of which its BIOS shows 2 boot choices. I set Optical Drive as first and then Hard Drive as second. Then in another selection in the BIOS I have to make the list of boot devices for #2 as USB Boot, then my 256GB SSD, and then my 90GB SSD.
I have my old Windows 7 build on my 90GB SSD as an alternate boot, my clean Windows 10 installation to the 256GB SSD, and with the USB as the first on that list the option to boot from USB Sticks.
Lastly to keep the read/writes to a minimum on my 256GB Intel SSD, I pointed Windows 10 to use the 90GB Agility 3 OCZ SSD drive for paging/swap space for virtual memory so that I kill off that 90GB drive before wearing out the memory cells on my Intel 256GB drive since I have 9800 hours of operation on the 90GB SSD with 51TB of writes and 174TB of reads according to crystaldiskinfo which i use to keep an eye on drive wear and a lookout for problems when monthly performing system health inspections and recording drive health. This drive has such a high read/write wear on it because i used it heavily for video processing of large video file projects that were recorded in fraps and then encoded into smaller avi files when i use to record gameplay for our world of warcraft guild runs for people to be able to watch runs they werent able to attend as well as see instant replays and learn from mistakes etc.