Doesn't matter if it's a "smart tv" - you'll be connecting the Roku as an external device.
Yes, it will need an HDMI cable to connect it to an HDMI port on your tv.
Roku has had a number of different models under the Roku 2 name, some of them with letter designations (Roku 2 HD, Roku 2 XD, etc.) issued from 2011 to 2015. They are outmoded and painfully slow by today's standards, and many of the more recently developed channel apps won't run on them.