not necessarily the rear window defroster, if you care, you may need to add a relay, vehicle dependant... Same goes for if you need a bypass (for remote start)
the 5901 is the replacement for the 5900. The 5901 has the LCD screen (newly designed remote and brain), LC3 technology and bunch more features than the 5900, go to directed.com for more info on it. Might need to google search for the 5900, not sure if its listed on directed.com anymore.
you'll need a DLPKGM, or XK06 or a GMDL-BP will work as well, for remote start.