1995 and older is 3.0, which is known for popping head gaskets. 1996 brought OBDII and the well loved 3.4L V6. Both are timing belt motors, but they are non-interference. The desirable 4.7L V8 is also non-interference timing belt.
The 4 speed auto is pretty robust. No worries there. V8 got a 5 speed, but again, robust.
I want to say though the big issue with the V8 is the shocks--well, maybe not a big deal, but more expensive if it has a fancy system. I know my brother has a V8 4Runner, and talking to him I think he plans on doing a lift when the shocks die.