2005-2010 were the most problem-prone years, even more so in the 2005-2006 years. The radiator cooler issue was one of the major problems; another was the upper timing chains. Some of the upper timing chains had links that were stamped with worn tooling that created sharp edges that would cut into the plastic tensioner faces. Because this is dependent on how much wear was on the tooling, if any, there is no specific mileage that this problem occurs. It will be evident by a "whining" noise that increases/decreases with RPM that gradually gets worse over time. It reminds me of the sound I used to hear from failing power steering pumps in old Ford vehicles of the '80s. It's a pretty big job to do, requiring the removal of the front timing cover and replacement of, at least, the upper chains and tensioner faces, but most will go on to do the primary chain, water pump, thermostat and related parts while it is open.
We also sometimes see low oil pressure at idle issues, caused by a failed oil gallery cover gasket located on the front of the rear timing cover (out of view unless the front timing cover is removed). 05-06 model years have an actual oil pressure gauges, so you can see the low oil pressure reading, but 07-and-later models have an oil pressure switch rather than a sender, so it essentially turns the oil pressure gauge into a "dummy gauge." Other signs of this problem could be the setting of P0011 and/or P0021 engine timing codes. The screws in the cover plate sometimes strip or break when trying to remove, so could end up resulting in needing a rear timing cover assembly for the repair, but not always.
Faulty fuel senders causing a P0463 code was another common problem during the first 5-years.
Front end knock due to the front sway bar hitting the crossmember wasn't uncommon in early R51's; Nissan updated the mount bushings, which is a cheap and easy fix. There were also some vibration issues at idle caused by bad motor mounts, but even with good mounts, there typically is a little idle vibration in these trucks.
There were some serpentine belt squeak issues in the first 5-years, addressed by a bulletin that instructed to check the power steering pump alignment and to replace the tensioner and belt, the latter which was updated to be 10MM shorter, which creates more tension on the belt. Gates refers to it as an "enhancement kit."
As this vehicles get over a decades old, the plastic parts on the heater pump assembly, especially those with rear HVAC, can get very brittle and break (it's happened to both of my R51's). I would recommend replacing it if the vehicle has 10+ years and/or more than 100K miles on it. We've also seen a few leaks on the aluminum coolant lines to the rear heater, caused by dirt and sand that gets under the rubber protector where they mount in the rear, which scratch through the pipes. Replacing the line assembly requires lifting the right side of the vehicle off of the frame, but in most cases, the lines can be repaired.
Some notes: the 2008 (and later) Pathfinders have a different nose than the 05-07. The nose has a steeper rake, so the grille, bumper cover and headlamps are different. In 2008-2012 model years, the R51 was also available with a 5.6L-V8, which is nice if you are towing heavy. The V8 did have some problems with exhaust manifolds cracking/leaking.
The brakes on the R51 aren't to everyone's liking, tending to be a little soft or requiring a lot of travel when braking. Sometimes a proper bleeding can improve this, but mostly it's inherent to the design. Better pads and rotors and/or stainless steel braided brake hoses make a difference. If you have 17" or larger wheels and a V6 (which has 11" rotors), you can swap over the 12" front brake rotors, front calipers and pads for a V8 Pathfinder, which adds a lot of improvement.
LE trim has an AUTO mode transfer case which isn't available on the other trims in the early years. It's similar to GM's old Smartrak II system, utilizing a wet clutch to automatically apply power to the front wheels when needed or anticipated, which is a nice feature to have. LE also comes with faux wood trim, leather seats, 17" and LE-specific wheels, among other things. SE-Off Road trim has additional skid plates, hill assist and hill decent control and larger diameters tires, among other things. Some SE models will come with a "fold-flat" front passenger seat, which, as described, lets one fold the seat back flat down against the bottom cushion, making more room if you are carrying something of length inside the vehicle, or need a place to put your lunch.
IMO, if you can afford it, best years to get are 2011-2012, as they have most of the "buggs" worked out of them. Good luck!