3.0s are excellent engines. They are not the most powerful V6 ever made, but they are durable. The only weak link is the cam synchronizer on 1995+ models. If it fails completely, the oil pump stops. Usually you will get plenty of advanced warning though (chirping noise), and it's easy to replace.
The 2.3 and 2.5 Lima I4s are excellent as well. Those are the ones with eight spark plugs. Changing the plugs on the intake side of the head is a bit of a challenge. Also, these engines use a timing belt, but they are non-interference. Lima I4s are very durable as well.
The Duratec 2.3 is the current I4 and is pretty good. There are many high mileage ones and it seems to be very reliable.
The SOHC 4.0 isn't a bad engine, but ones built before 2004 are known for timing chain noise. Ford has revised parts to fix it. Also, the SOHC 4.0 is more prone to head gasket leaks than other Ranger engines. I'd rather have the pushrod 4.0 personally.
The SOHC 4.0 has been used since 2001. The pushrod 4.0 was used from 1990 to 2000. The 3.0 was used from 1990 to 2008. The Lima I4s were used from 1983 until 2001.5, and the Duratec I4 has been used since 2001.5.
If your son is looking at automatics, 1995+ trucks will be the best. They have stronger automatics than older ones. Also, the 5R55E 5-speed auto became available in 1997 behind the 4.0. All engines got 5-speed autos in 2001.