If you want to invest in new hardware, I would suggest getting a mini PC. You can get these refurbished for under 100€. Add a wireless keyboard with touchpad (Logitech K400 or Microsoft Media Keyboard) and install Linux+Kodi.
You can select Kodi in your login manager to start straight into Kodi, so you dont have to mess with a desktop environment if you just want to watch a movie.
KDE is also developing a desktop environment for TVs called Plasma Bigscreen, although I haven’t tried this one.
And unlike devices with a locked down, proprietary OS you can run any desktop application or game on it (if the hardware supports it).