Quote Originally Posted by markw
While you might think it's a fairly simple task, the reality is that you're gonna need some fairly husky speakers and a considerable amount of power to drive them. This doesn't even take into account any mixing boards you might want.

You're best bet might be to solicit some opinions from local sound reinforcement rental companies in the area to see what they would recommend. That might give you some idea of what you're up against.
I agree, I think this is not necessarily the wrong venue for advice, but you might be better served elsewhere.

As a musician I have experience with filling a large space with sound. Your best bet will be to consult with a local music shop, OR try any local company that deals with audio logistics for events, concerts, festivals, etc.

These guys may have a sales angle, but I think a good and reputable business will give you the info you need, understanding your budget constraints. The big difference with going to a local pro is that they can view the area in question, which we can only try to imagine.

On another note, I think some of the suggestions so far have been pretty crafty considering the budget. Great job guys!