For the hardware, there are many different "TV" cards on the market that can output video. Take a look at ATi and nVidia since these are two very popular brands (there are undoubtedly many others as well). Also be sure to look for VGA-to-TV conversion tools.
To output multiple channels at once though, you'll more likely want something like a "cable TV headend" unit that can take multiple inputs and push them out on a single coaxial TV cable (Cisco bought Scientific Atlanta many years ago -- Scientific Atlanta produced this "headend" equipment, so I guess Cisco probably does this now).
I suggest you learn to do a little bit of shell scripting or Perl scripting (Perl would probably be better for this purpose) because then you can customize it to get it to work exactly the way you need it to. Your Perl script could read the list of files in that folder, and control your streaming application.
The VLC project provides excellent solution for this sort of thing, although I've only used it for recording from video cameras I've read that it can provide video server functionality as well (e.g., multicast RTSP, etc.); it should be able to send audio too (I assume because you wish to run a radio station also?):
VLC / VideoLAN - Projects
Both Perl and VLC are free and open source. There may be tools out there that can do what you need, but if I were doing this then I'd want to customize and own the source so that I could make it work exactly the way I need.