Hello, BeanP. I can build site which you requested, I have lot of experience with Gmaps. For you, wining combination is Gmaps v3, + Joomla CMS with custom component with all requested features, + Jquery library for eye-candy, smooth transition on the map.
From my previous experience, my recommendation for you is not to use markers as objects. Use custom overlays, instead, because map will be overloaded with too many markers.
For core function on the map, you should use pure Javascript instead of JQuery, that will make map "lighter" for dragging. Communication between objects on map will be AJAX-ified, of course.
Timeframe is large, because for such applications, we need weeks of testing and "fine-tuning".
If you need HQ application, feel free to contact me.
Thank you.