Do not under any circumstances buy a plasma. Apart from the fact that they weight a ton, they are affected by "screen burn" when you display the same or a similar image - like a PPT template for a long time. Screens installed in a place of work are often left on for extended periods - sometimes overnight or at a weekend.
Some TV's will allow you to run a PPT from the USB socket however the features vary between manuafacturer and model.
I'm a tech in an FE college, and apart from projectors, we have Samsung and Sony domestic TV's (46"/52"/55") and professional NEC monitors (used for electronic signage).
The Sony's will only show a JPEG slideshow when a USB pen is plugged in, and if there is anything else on the pen - Folders, Word's/Excel's/MP3's etc. it won't show anything.
The Samsungs (of which we have many different models of varying ages) are a bit different. They still struggle with anything other than JPEGS, but if you convert a PPT to a JPEG (and resize it if it has been created 4:3), you can view the PPT as a slideshow. However, some of the Samsungs change slide every 10 seconds, others allow you to choose the refresh rate 5-10-15-20 seconds, and one model - only one - allows you to manually advance a slide. That is the only viable way you can use it for a presentation.
You still have the problem of having to convert it to a JPEG.
As another poster says, it would be a lot better just using a laptop or a PC.
One other thing, as well as HDMI, you must ensure that the screen has a VGA input - not all screens have them as they are on their way out, but a lot of older laptops that are issued to employees do not have HDMI outputs.
You say that you don't need to use the screen a TV so a monitor would suffice.
Look at the NEC Multisync range. A bit pricey, but built like tanks and built to last.