I have some good news and some bad here.
First I would recommend you stick with the Samsung 850 EVO Vs the Pro version. You'll be just fine with the cheaper drive. I would also recommend you bump up to a 1 TB drive if you are planing on using more than one OS here. You'll want the larger space for Windows paging & virtual RAM.
As to the systems SATA ports I/O speed. Yes, you'll need to deal with it here. Before you alter anything you'll need to make sure your systems firmware is up to date. Follow this Apple T/N: About EFI and SMC firmware updates for Intel-based Mac computers. Make sure you at this or newer level. The Samsung 850 EVO has auto SATA speed sense technology so the drive will match up with the systems SATA ports I/O speed in this case SATA II (3.0 Gb/s).
I would also recommend upping the RAM as well from the standard 4 GB to at least 8 GB.
OK now the bad ...
As you're replacing your HD you'll need to make up the lost HD thermal sensor the system requires. Luckily you have an out here you'll need this OWC In-line Digital Thermal Sensor for iMac 2009-2010 and you'll need this OWC 2.5" to 3.5" Drive Adapter Bracket.
Not so bad after all ;-}
Here's the last thing, follow this IFIXIT guide: iMac Intel 21.5" EMC 2389 Hard Drive Replacement