Replacing the 2011 27" iMac's original hard drive with an SSD isn't as straight forward as upgrading memory, but can be done as follows. Disconnect all cables from your iMac, double check power cable is disconnected. Remove display glass with rubber suckers. Remove 8 screws holding in LCD. Carefully pull back the LCD from the top, just far enough to slide your hand into the gap. Do not stress any cables. Carefully disconnect 2 cables from the logic board and 2 from the backlight board (top left corner). Be careful removing the wide cable from the LCD, it is held by a locking bar. I have seen some users let the LCD fall back and rip the connector off the logic board, the cable is short. Carefully put the LCD to one side. Take outside, use canned air to blow out dust, specifically from the 3 fans and 2 finned heatsinks. Removing 2 screws and pulling the 2 sata cables removes the existing hard drive. Fit your SSD in a 2.5" to 3.5" hard drive adapter, the Icy Dock one sold by Crucial is good (as are their SSD's). Transfer mounting hardware from old hard drive to the 2.5" to 3.5" adapter. Decision time, as the old hard drive had an internal temperature sensor connected via a custom power sata cable. If ignored your 3 fans speed will ramp up to a very noisy and unbearable full speed over 10 - 15 minutes. Options are to use fan speed control software, an external temperature sensor kit, or add your own sensor by using a 2N3904 transistor linked as a diode and integrated into the 2 additional wires in the sata power connector (the larger of the 2 connectors). I prefer the latter option, simple and economical. Cutters, soldering iron and small bore heat shrink tubing req'd. Reassemble and clean any accumulated dust from LCD and inside of display glass. Then either clone your old HD to the SSD or install an OS and start from fresh, recommended if you have just bought the iMac used. Install from a USB installer created earlier. In simple terms, job done.
Potential problems to look out for when buying a 27" 2011 iMac. Dust contamination within the LCD, easily spotted on a white display, looks similar to a cloudy sky, but usable. Not easy to spot, potential graphics card issues that were previously covered by a repair extension program for up to 4 years from the original purchase date, so in 99.999% of cases not now covered. Many graphics card issues can be intermittent, and can make the iMac unusable. Both not considered economically viable to spend money on. Optical drives are common to fail, no issue as if required a Samsung external is cheap and effective. Consider the newer thin style 27" iMac, either the older late 2012 model onwards, or new. They are so much better, runs cooler and are very reliable, although like all iMac's, was not intended to have it's hard drive user upgraded.
. and it ran Lightroom, Photoshop and Final Cut Pro just fine. OK compared to the new 5K replacement it was a bit slow, but to be honest if it wasn't for the increase in video work I would have been hard pushed to justify it's replacement So the model you are looking at should be fine.