can we install Samsung 860 EVO 1TB 2.5-Inch SATA III Internal SSD (MZ-76E1T0B/AM) in a late 2011 iMac ?

Here is a recent post from someone that installed an EVO 850 into an iMac without the heat sensor from OWC's installation kit, and the drive died within a very short time. There was no late-2011 27 inch iMac, but there was a late-2011 21.5 inch iMac. Here is an OWC installation video on how to install an SSD into a mid-2011 21.5 inch iMac. Not as simple as just yanking out the existing rotational drive.

Consider the purchase of a Thunderbolt 1/2 to USB3 cable, and housing the EVO 860 in an external drive enclosure.

Yes. You need to buy a special adapter cable from OWC to keep the fan from going ballistic.

As Kappy mentioned you can, however if it were my computer I'd install an OWC SSD and include the kit.

No, but it is a complete kit and tends to just work rather than trying to cobble together a cheap SSD and put it in your iMac. Sometimes when trying to save money users spend much much more because they don't get the right components and/or simply don't know what they are doing.