This may be blasphemy but -
The fact of the matter about all outdoor theaters is that you're usually limited by outside factors. Your neighbors (and sometimes your wife). We can all set up a surround system if everyone sits in the right seats and your neighbor doesn't mind you're bumping subwoofer at 11pm. For simplicity I go with stereo and for the neighbor factor I lose the sub. That way I can do it more than once a year. Also consider how your speakers are pointed. If your speakers are pointed at you sitting by your house, the sound reflects right back off your house. Try it out and see for yourself. I sometimes put the speakers in back of me (yeah I know) because the sound in my case, goes away from my neighbors house. It's actually kind of amazing how much louder I can be because it goes the opposite way. Let's face it, people move around, talk, drink and eat while kids run around. Usually we are the only ones that know, or notice, the sub and the surround isn't there. We aren't screening an oscar movie for the academy. I'm just saying. A simple setup also goes a long way when (not if) weather rolls in. I assume I'm not the only one that has a radar app that you check a few times?
If your buying a new projector, buy more lumens than you will use. Running it in "Eco Mode" makes your bulb last 20-40% longer and reduces noise and heat.