Hi John4343,
I don't have any direct experience with what you want to accomplish, but you probably want to purchase Multimedia Presentation Software for a computer, then connect the TV set to the computer.
You can probably use Microsoft Powerpoint to do some basic sequencing of video and messages. To change your message or videos would simply be a matter of editing the Powerpoint file.
Assuming you will use an LCD TV set with HDMI input, you can purchase a video card (for the computer) that has HDMI output. However, be aware that HDMI has cable length limitations. You can overcome the cable length limitations of HDMI by using devices like these: http://www.atlona.com/Atlona-1080p-HDMI-Extender-over-Single-Cat5-up-to-200ft-NEW-p-17468.html
Here's a fairly comprehensive list of Business Presentation & Multimedia Authoring Software for Windows: http://graphicssoft.about.com/od/businesspresentationwin/Business_Presentation_Multimedia_Authoring_Software_for_Windows.htm
Please ask questions, but you basically have two issues to contend with: 1) Software for a computer that will let you sequence messages and video and 2) Determine how to connect the TV set to the computer.
BTW, according to this vendor you can connect the TV to an IP network and display video and messages, but it will require more hardware and will be more expensive: http://www.viscomsoft.com/products/avmanager/