Do I need to sand my entire deck before staining?
I wouldn't belt sand any outdoor deck, especially with a commercial belt sander that your talking about.  More damage will ensue.  Take the $60.00 you'll spend on the rental and buy a Makita Palm Sander.  It takes a 1/4 sheet of sandpaper to load onto the pad to sand.  Buy some 80 grit sheets and some 120 grit sheets.  Grab a beer and get down on your knees and feel for rough spots, sand lightly with the 80 grit on all the rough spots on the deck.  The electric sander will make light work of sanding the deck without going to far.  Then swap out for the 120 grit and hit all the spots again and your ready to stain/seal once you do your clean-up.. that size deck may take you 2 -3 hours total, good luck.