Honestly, Netflix and Prime are what I use, you already have the internet. Both services offer a free month. I don't know if you have a smart TV ready to go, but if you have the capability give it a shot, SlingTV has 7 days free. Try it out, It will cost you nothing and your still set up with cable. No lose situation.
Roku 3 are 99.99 and yes, a one time buy. I use Amazon fire sticks for the bedrooms 39.00. Roku streaming stick is 49 Either device will make any t.v. with an HDMI a smart tv both are one time buys.
Scroll down to the comparison chart IMO fire tv stick is best bang for the buck.