Sounds like you do need to do a fair bit of research. If you're looking at diesels, the '02-'06 NM-NP are the ones I'd avoid. They're comfortable, pretty frugal, but that generation of 3.2 4M41 runs a VRZ style injection system, and those fuel pumps are hideously expensive to replace/rebuild when they go, and they're all at that age where it's a matter of time. The '07-> 4th gen brought in Common Rail injection, which is a bit more reliable, and not as expensive to upkeep as the older injection system.
As you noted, the earlier 4th gen (NS) had a pretty mediocre DPF system that was prone to failure, but this was only the automatic NS. NS manual didn't have a DPF.
The '09 onwards from the NT brought on a few decent upgrades, like stronger automatic transmission (out with the crappy Jatco and in with the stronger Aisin A750F) and power/torque figures went to 147kW/441nm. The earlier NT auto had a "DPF", but they were basically a glorified catalytic converter and didn't actually have any problems.
NW didn't bring too many mechanical changes other than NO variants had a DPF, and the rear difflock was standard across the range instead of an option.
Early NX wasn't much more than a facelift from the NW, being mechanically identical up until MY17. NX MY17 had the DPF reintroduced, and also had taller diff gears (from memory 3.6ish instead of the previous 3.9:1) which hobble them that little bit more offroad IMO.
Anyway, for $15k the most you're probably going to get is an NS. Sure, the auto had the dreaded DPF and mediocre Jatco auto, but otherwise the rest of it was pretty reliable, and DESU as long as you weren't tuning big torque figures the Jatco was reliable enough. If you can find a manual NS, then it's happy days. You might find a high mileage NT, and as long as it's been looked after it should prove reliable.