Drive replacement like this is doable but you need a 2.5 to 3.5" drive adapter. The drive will not fit in this computer as-is. You will need this adapter to install the hard drive. I have also supplied the guide below.
If you really want to use Seagate here's the drive you want. As this is a 2015 revision, count on this drive being discontinued. I'm simply including the link as a reference.
iMac Intel 20" EMC 2133 and 2210 Hard Drive Replacement
Okay, now for the revised answer from a new era.
No, don't use Seagate. At this point in time, their drives are absolute junk and simply do not last. I also tried a SSHD before and I'm not impressed. The only thing Seagate is good for is target practice these days.
What I would do is get a WD Black drive and a optical drive caddy for the SSD.
You will need one of these to replace the DVD drive and then you can install an SSD into this and load the OS here. Get a Samsung 850 Pro SSD.
Without specifics on this, I can only guess what computer you own. As such I am not linking guides. You need to find them.