Sorry, any of the monitors I would recommend would put you well over budget with just one speaker.
If your looking to spend under $300.00, you might try one of the many surround pkgs available at places like Comp USA or Best Buy.
Are you ready for the increase (big time) in the time youll need to spend on sound tracks to build a 6 channel field?
Do these event videos really require it?
If you create the discreet mixes satifactorily, what will u use to encode them into an ac3 file?
Are you going to also make a LtRt mixdown for those without decoders.?
Presumably everything youre talking about will go to dvd. If not how will you create a Dolby e mix.