Don't buy the Latest Roku 1
I got one as it had okay reviews, and can play Netflix, iPlayer, Youtube and Plex.
But upon trying it, the interface is extremely laggy, and it can't even play HD videos on Youtube or iPlayer smoothly, I paid £30 for it, and have no use for it.
I then bought a Fire TV Stick to replace it, and the interface is really smooth, it plays iPlayer fine, but best of all, you can sideload Kodi onto it, and it plays videos from the web on there really well. (I used a version called SPMC which contains some fixes for the Fire TV too.)
We also have a Chromecast, which works well for Youtube, and streaming local photos and videos from an Android device, and it works really well when using an app like Videostream (I'm sure other apps work in a similar way) to stream local mp4/mkv files of TV shows/movies from an Android device, and it's my favourite solution to play a downloaded video on the TV.
(Apparently Videosteam also has a Windows version which lets you cast files to the Chromecast, but I haven't tested it, so I can't comment on it, but it might be worth a try if you don't want to use an Android phone/tablet).