Hello Brian
I have been looking at projectors for quite a while, Lyn may be doing a talk for the local garden club.
As you say a complete minefield.
I think you need to look at a table for each projector which shows the distance away from the screen for the projector to give the size of picture you need.
Price of spare lamp for the different projectors can vary greatly, £250+ is an average price
And of course brightness of the projected image is a big decision. You have mentioned the ambient light in the room as being one factor, the type of screen is another, and the distance away from the screen.
If we do have to get a projector I reckon we would get one with a light output of 4000 to 5000 ANSI Lumens for a hall, and that would cover pretty well any hall should Lyn decide to do more talks, but at about £50 ish per talk thats going to be a lot of talks to recoup the money for the projector, and by the time you have got the money back it will be time to get a spare lamp, ready for the time it fails during a talk. Which also brings to mind, how easy is it to change the lamp, some are easy some not.
So really I am not much help, guess the best way is to get a demo, if you can find an outlet nearby.
Malcolm