Seeking advice on setting up outdoor event sound system

I have setup sound systems for indoor spaces, but I am now tasked with setting up a system for an outside venue.  The space I am working has an area of about one acre (43,560 square feet).  The space is about as wide as it is long.  The area is mostly surrounded by growths of large cottonwood trees.  I am dealing pretty much with an open space within the trees.

The requirements for what the system will be used for are pretty minimal.  I only need a system for an announcer and other speakers.  The performers will be using their own portable systems. 

The layout of the venue is that the performers will be in the middle of the space, with the audience sitting in a circle around the performers.  The announcer has a stand on one side of the space, facing the center.  I am thinking about two speakers being placed on either side of the announcer stand.  But there are shade structures at the site which could be used to safely run cables to more speakers in the area.  I don't know the issues with having a "surround sound" system outside and if there would be feedback issues with the announcer's mic.

 The system would only be in place during events and taken down when an event is over.  The speakers would not have to be absolutely weatherproof as they are not to be permanently installed.  But there are bugs and intermittent rain shower are an issue.  So some weather protection in the speakers would be helpful.

I am thinking of wanting to use tripod base, portable speaker poles.  This would allow me the ability to experiment with speaker placement.

I probably will never need more than four channels.   I am leaning towards using a powered mixer, rather than a separate mixer board and amp.  This would simplify moving the system around and space it would take up.

So does anyone have suggestions as the wattage of amp power that would be required to fill the space and make the announcer understandable? Also, suggestions for specific models of speakers, mics, and powered mixers would be appreciate. 

I can send along more details about the venue and usage as needed.

 I only need a system for an announcer and other speakers. The performers will be using their own portable systems.  

You answered your own question then.
But what are the specs for a system that would be best for even just the announcers in this setting?  The devil is in the details and you sent me no devils to deal with.
Tripods might not be stable on dirt.  You don't want things falling over.  Also, speakers down low are lousy for the people in the front or back.  You need more height.
Since you want some sort of roof anyway, consider a beam or pole to hang everything from.