I would definitely want to know about depth. You can use a slip bobber rig cast from shore to get a pretty good idea. Retention ponds by their very nature tend to collect runoff from everyone's yards, including pesticides and fertilizers. Maybe that explains the neighbors' use of copper sulfate, in order to combat an algae problem.
By the way, there are better chemical alternatives these days.
In some instances that I am familiar with, neighbors cared far more about aesthetics than fishing, where retention ponds were concerned. Hopefully you will find your new neighbors more open to managing with an eye towards fish, rather than just completely weed free.
I would begin fishing it to get a feel for what's down there, and to establish depth. Here in Indiana, you need to be 8-10' in order to offer protection against winterkill, although aeration will allow you some leeway, provided the neighbors are on board with the idea....might be a tough sell. The fact that the pond has killed in the past needs to be explored, and hopefully remedied. No sense stocking fish if they will perish in a year or two.
