The cheapest is to buy some android box with armlogic processor and install coreelec on it. You can do it for 20 bucks, then you have a kodi oriented linux distro on your tv.
Though I prefer to straight up connect my laptop to the tv with a small remote keyboard and have full computer functionality. I'm looking to change the laptop for a miniPC when the laptop finally breaks down. I would use a normal DE. Nothing specially suited for smartTV usage. But you get used to it pretty quick.