I totally wouldn't buy that stuff. First off, it's not a good deal. Used car audio equipment generally has a market price of 50% of retail, no one should pay more than that. From what you have listed, it looks like you should be paying about $200 for all of it.
But that isn't really the point, it doesn't really matter how much you spend, it's what you buy. It doesn't matter if it's $200 or $2000, if it's crap, it's gonna suck. I'm familiar with PowerCoustic, and I've never cared for it. If you were to spend another $150, you could have a system that would sound a million times better, and last ten times as long.