I just purchased an older SQN 3M audio mixer to use with my XL2. It has some sort of open housing on the top of it that appears to be for the the batteries along with some extra connections of some sort. I'm attempting to shoot a higher quality documentary and am wondering if this was the right decision. I believe I would have been better off with a sound devices 302 but these things don't pop up that often used on ebay. Any comments?

Well, the fact you don't see them (SD302) very often on eBay is an indicator that folks that buy 'em tend to keep 'em.