Any Blackmagic device you get will come with Blackmagic Media Express. It's the software you'll use for capturing the input through the interface. It will work great if you have a fast enough drive to capture the source resolution — spinning disk drives tend to max out around 135-150 MB/s no matter what interface you have: USB3, TB, etc. so you'll need a fast drive like a RAID or SSD that will capture better than about 350MB/s or so for 1080 HD. Since you're talking about S-Video (SD) you should be good with a reasonably fast usb3 drive (you take your chances with spinning disks... they can be as slow as 65MB/s even if they are usb3, tb, etc...)
Media Express has a log and transfer feature like the old FCP7 did (the transferred video is compatible with FCPX, you'll just have to import it separately). If your media source equipment has HDMI output and your Mac has Thunderbolt, you might want to check out the Blackmagic Mini-Recorder (do not confuse it with the "Mini-Monitor" which is a completely different device and is not designed for capture.) It will save you some money.