Sounds like what i would like to accomplish one day.. As for what code to start with, its always good to start with either a sample sketch for certain sensors so that you can test and debug then move to your master sketch.
For me that helped insure that each sensor worked individually then combined into the master sketch and do progressive testing as you add so that you can begin to see your over all automation at work.
By the sounds of what you want to accomplish the mega may be the board to start with to allow all the connections, but ive only worked with the uno and mega, so not sure if that is truly the best board to use.
Others will def chime in.. but good luck and post back update or if you have other questions...