The Pathfinder has been through 3 generations. They were from
1986 to 1995, 1996 to 2004, and 2005 through the present. The platforms for those generations was the WD21, R50, and R51, respectively.
The Pathfinder is sold as either a rear wheel drive or four wheel drive SUV. The first generation was sold as either a 2 door or 4 door SUV, while the 2nd and 3rd generation models have only been sold with 4 doors.
The first generation Pathfinder was the only one to be sold with an I4 engine. The 2nd and 3rd generation Pathfinders are all sold with either V6 or V8 engines. That's good news, because this vehicle is way too big for a 4 cylinder engine. I would advise buying a used Pathfinder with the I4 engine, especially if you plan on using it for carrying a heavy payload or towing anything more than the hitch.
The trim levels on these SUVs include the XE, SE, and LE.
If you are planning on buying a used Pathfinder, the best place to buy is on eBay. They have lots of listings on various models, some of which might be located near your area. There are also lots of parts on there.