As for add-on packages, there's another important aspect to consider. Add-ons almost always seem to be a few months (at least) behind AutoDesk. You won't be able to upgrade AutoCad because your add-on hasn't been updated to be compatible, yet. With EaglePoint, I recall we couldn't upgrade...
Another thought on storing the water for later use. It might get very nasty before you use it since it isn't being treated (such as chlorination).
If the area is very flat, it might be difficult to accomplish very much since the neighbors' yards will drain right into your nice new drainage...
