-Pressure regulator for your fresh water hose
-Second fresh water hose (sometimes you are to far away from the hydrant)
-Box of rubber gloves or hand sanitizer for dumping tanks
-clear elbow for sewer hose (would rather "see" when it is done than the other way to find out)
-leveling blocks or chunks of lumber for "side to side" leveling
-Adaptor(s) for power cable 50/30/15
-"Y" connector for fresh water hook-ups
-and the list goes on but these are the basics.
As for de-winterizing, too easy. You literally run the pink stuff out. Depending on your tank size, add bleach to your FW tank. Fire up the pump and run the pink stuff out using your FW tank and water pump. When pink is gone and you smell bleach, stop, let it sit overnight then flush the bleach solution out. De-winterized and sanitized. Now switch the by-pass back to normal use on the water heater. As to "when" all depends on your temps overnight. If you are hovering around freezing at the coldest part of the night then go for it. More than a few hours below 32 and you may have issues. Hope this helps!