If you want to stay old school a cat 3208 will give you good torque for pulling a trailer. THe 3208 is mechanical so no electrical issues. The newer CAt 3116 is also a good engine, may take a little more to hook it up and will cost a bit more to buy. Adding transfer case and front driving axle is doable, but a lot of work. Usually better to buy a 4x4 truck to start with than convering an older 2x4.