2.3 and 2.5 Ford Rangers are near impossible to kill (they aren't DOHC that I have ever seen)
Not great on power, but they will almost always go over 300,000 miles
2000 and older 4 cylinders are SOHC. In mid-year 2001, Ford switched to the DOHC Duratec 2.3L.
I agree that they're super hard to kill. The Duratecs are very stout too.
3.0L have great reliability, not so great gas mileage.
OHV (pushrod) 4.0L (2000 and older) have great reliability and even worse gas mileage, good power though. The intake manifold gaskets tend to go out once they get 200k on them (I've personally had it happen on 2 different engines. Same failure in same spot. Both engines had over 200k on them with no troubles up to that point), other than that, they're great.
SOHC 4.0 (2001-2011) has lots of power, same mileage as the pushrod 4.0. They had some timing chain issues prior to 2004. 2005+ should have updated parts from the factory and be good to go. They have plastic thermostat housings that like to crack and leak, but that can be solved permanently with an aftermarket aluminum housing.