Thanks for that Milo. I'm so out of touch I had never heard of Displayport!
It seems like the BenQ W710ST DLP projector would meet the requirements and is at the top end of the budget with the river people: 1200x720, 2500 ANSI lumens, HD ready and with 2x HDMI, RCA, Component, S-Video, D-Sub, USB, RS-232, Audio-Out.
Seems we would need to get a Displayport to HDMI adapter and then that combination of interfaces should cope with all modern and not so old laptops. Audio isn't too much of a problem as I understand from SWMBO that the projected pictures supplement the talk/class instruction and audio is not usually used.
Any other comments greatly appreciated!