Everyone is right: $600 is not enough for a usable PA. You will be disappointed with anything at that price point (e.g., used behringer...). You'll hate it, then you'll have to sell it to trade up, and lose $ and time in the process.
So, this is gonna sound crazy, but if you just want to hear the singer -- one singer -- consider the JBL PRX612m or PRX615. These are 1000w powered cabinets and you can plug the mic directly into the speaker (this is blasphemy -- but it will work). I've been watching eBay, and you can get the prx612x for $600 (the PRX615 is maybe $800). Of course, that's just one cabinet; you need to save up for another one. But these cabinets are the real deal -- very loud, very clear, no need to trade up -- and they will hold their value reasonably well.