Budget Projector for Variable Lighting

I am working at a summer camp and we are thinking of getting a new projector for watching movies in the evening. The projector is going to be set up under a building which is partially on stilts (there is a wall behind the screen but it is open on the sides). The movies will be started just after sunset when it is still fairly light out. The area will of course get darker as the evening progresses. It will never be pitch black however because directly behind the projector is the snack bar. The snack bar doesn't put out too much light (you couldn't read a paper in the seats) but it does of course put out some.
Before you lecture me, I am well aware that this is a challenging setup for a projector. I was thinking that a bright projector with a dark gray screen (it will be a diy painted screen) might be the best solution.

I am looking at the Optoma HD66 as that has 2500 lumens. The Panasonic pt-ar100u looks like it might be ideal since supposedly it will adjust the picture in response to changing lighting conditions but I think it is out of our budget range.

What are your thoughts on the setup as a whole? Is the Optoma HD66 a good choice? Any other suggestions for budget projectors that fit our needs?

BTW the current setup we have is an Epson Powerlite S5 (an 800*600 data projector with terrible picture) projected onto a gray painted tongue and groove wood wall with obvious wood grain and sizable gaps between the planks. Almost anything would be an improvement.

I'm thinking 100"

Throw range doesn't matter, I will be mounting it on the ceiling and have plenty of room to play around with