My classroom is equipped with video display for demonstration purposes. I chose to use a cheap video camera that cost about $120 twelve years ago. I mention it because I have compared it to ELMO document cameras costing in excess of $1000 and I prefer my set-up in every way. Expensive document cameras seem to be less suitable for video. The frame-rate on the ones I have seen is low enough that the video image appears stuttered. A pencil shading demonstration, for example, would show my hand jumping abruptly around the screen like poor stop-motion animation or something. The camera video is much smoother. Additionally, it can double as a camera to record video. As for longevity, I seldom ever turn mine off. I literally turn it on in August and leave it on through the weekend all year. I turn it off before school is out for holidays..but then, only if I remember to. So its been running for twelve years and shows no degradation in the image or camera sensitivity.
My video outputs to a 48″ flat screen but it would work just as well with a projection display. I rigged my camera over a podium using some hardware from an old tripod.