King Cab or Crew Cab ?
Big Tow package (hitch, relays, big mirrors, tranny temp gauge) ?
I would replace the spring shackles with 2008+ shackles and make sure it had a decent set of LT tires and shocks (like Bilsteins). The ATF rubber cooler lines are known to fail, so I would replace those with ATF cooler rated hose and clamps. OEM Cooling is pretty good, although the stock radiator is part plastic and prone to cracking. You can get a better all aluminum radiator from CSF. Some people bypass the ATF cooler in the radiator and add an aftermarket cooler to avoid any possibility of mixing ATF and antifreeze if the radiator fails. Make sure your brakes are good, you can upgrade with aftermarket brakes like Frozen Rotors/ Hawk LTS pads. Some people have upgraded to the larger 2008+ front brakes, which requires 18" or larger wheels and hub spacer rings.
I think the guys that tow heavy use equalizing sway control hitches and some of them run airbags. You need to pay attention to tow rating, tongue weight and GVWR, which you can find at nissanhelp.com